feat: Add security hardening, agentic coding infrastructure, and test framework

- Add SECRET_KEY and ENCRYPTION_KEY validation on startup
- Implement security headers middleware (X-Frame-Options, CSP, HSTS)
- Add CSRF protection middleware
- Create comprehensive GitHub issue templates and PR template
- Add Makefile with common development tasks
- Configure pre-commit hooks (black, ruff, mypy, bandit, detect-secrets)
- Create docs/CODING_PATTERNS.md with best practices
- Create docs/ERRORS.md documenting all error codes
- Add Architecture Decision Records (ADR) for Celery and Fernet encryption
- Create CHANGELOG.md for version tracking
- Set up pytest test infrastructure with fixtures and factories
- Add sample unit tests for security and config validation
- Create CI/CD workflows (test, lint, security)
- Add comprehensive TODO.md with milestones and progress tracking

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Bug Report
+about: Report a bug to help us improve
+title: '[BUG] '
+labels: bug
+assignees: ''
+---
+
+## Bug Description
+<!-- A clear and concise description of what the bug is -->
+
+## Steps to Reproduce
+1. Go to '...'
+2. Click on '...'
+3. See error
+
+## Expected Behavior
+<!-- What you expected to happen -->
+
+## Actual Behavior
+<!-- What actually happened -->
+
+## Environment
+- **Component**: [e.g., Backend API, Worker, Docker, pop3_forwarder.py]
+- **Version**: [e.g., v1.0.0, main branch]
+- **Deployment**: [e.g., Docker, Kubernetes, local]
+- **OS**: [e.g., Ubuntu 22.04, macOS, Windows]
+- **Python Version**: [e.g., 3.11]
+
+## Logs/Error Messages
+```
+Paste relevant logs or error messages here
+```
+
+## Additional Context
+<!-- Any other context about the problem -->
+
+## Possible Solution
+<!-- Optional: suggest a fix or workaround -->
+
+## Related Issues
+<!-- Link any related issues -->

.github/ISSUE_TEMPLATE/feature_request.md

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+---
+name: Feature Request
+about: Suggest a new feature or enhancement
+title: '[FEATURE] '
+labels: enhancement
+assignees: ''
+---
+
+## Feature Description
+<!-- A clear and concise description of the feature -->
+
+## Problem Statement
+<!-- What problem does this solve? -->
+
+## Proposed Solution
+<!-- How would you like this to work? -->
+
+## Alternative Solutions
+<!-- Any alternative approaches you've considered -->
+
+## Use Case
+<!-- Describe a specific scenario where this would be useful -->
+
+## Implementation Notes
+<!-- Optional: Technical details, API design, architecture considerations -->
+
+## Acceptance Criteria
+<!-- How would we know this feature is complete? -->
+- [ ] Criterion 1
+- [ ] Criterion 2
+- [ ] Documentation updated
+- [ ] Tests added
+
+## Priority
+<!-- Low / Medium / High / Critical -->
+
+## Related To
+<!-- Link to roadmap items, other issues, or discussions -->

.github/ISSUE_TEMPLATE/test_needed.md

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+---
+name: Test Needed
+about: Identify code that needs test coverage
+title: '[TEST] '
+labels: testing, help wanted
+assignees: ''
+---
+
+## Code to Test
+<!-- What code needs test coverage? -->
+- **File**: `path/to/file.py`
+- **Function/Class**: `function_name` or `ClassName`
+- **Lines**: [e.g., lines 50-100]
+
+## Current Coverage
+<!-- What's the current test coverage for this code? -->
+- [ ] No tests exist
+- [ ] Partial coverage (describe what's tested)
+
+## Test Type Needed
+- [ ] Unit tests
+- [ ] Integration tests
+- [ ] End-to-end tests
+- [ ] Performance tests
+- [ ] Security tests
+
+## Test Scenarios
+<!-- List specific scenarios that should be tested -->
+1. Happy path: 
+2. Error handling: 
+3. Edge cases: 
+4. Boundary conditions: 
+
+## Dependencies
+<!-- What needs to be mocked or stubbed? -->
+- External API: 
+- Database: 
+- File system: 
+- Environment variables: 
+
+## Acceptance Criteria
+- [ ] All scenarios covered
+- [ ] Edge cases tested
+- [ ] Error paths tested
+- [ ] Coverage increased by X%
+- [ ] Tests documented
+
+## Related Code
+<!-- Link to related functions, classes, or issues -->

.github/PULL_REQUEST_TEMPLATE.md

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+## Description
+<!-- Provide a clear description of the changes -->
+
+## Related Issue
+<!-- Link to the issue this PR addresses -->
+Closes #
+
+## Type of Change
+- [ ] Bug fix (non-breaking change which fixes an issue)
+- [ ] New feature (non-breaking change which adds functionality)
+- [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
+- [ ] Documentation update
+- [ ] Refactoring (no functional changes)
+- [ ] Performance improvement
+- [ ] Security fix
+
+## Changes Made
+<!-- List the main changes -->
+- 
+- 
+- 
+
+## Testing
+<!-- Describe the tests you ran -->
+- [ ] Unit tests added/updated
+- [ ] Integration tests added/updated
+- [ ] Manual testing completed
+- [ ] All tests pass locally
+
+## Security Considerations
+<!-- Any security implications? -->
+- [ ] No security impact
+- [ ] Security scan passed
+- [ ] Credentials properly handled
+- [ ] Input validation added
+- [ ] Authentication/authorization checked
+
+## Documentation
+- [ ] Code comments added/updated
+- [ ] README updated
+- [ ] API documentation updated
+- [ ] CHANGELOG updated
+- [ ] Migration guide created (if breaking change)
+
+## Checklist
+- [ ] Code follows the project's style guidelines
+- [ ] Self-review completed
+- [ ] No new warnings introduced
+- [ ] Tests added for new functionality
+- [ ] All existing tests pass
+- [ ] Documentation is up-to-date
+- [ ] No secrets or credentials committed
+
+## Screenshots (if applicable)
+<!-- Add screenshots for UI changes -->
+
+## Performance Impact
+<!-- Any performance implications? -->
+- [ ] No performance impact
+- [ ] Performance improved
+- [ ] Benchmarks added
+
+## Deployment Notes
+<!-- Special deployment instructions? -->
+- [ ] No special deployment needed
+- [ ] Database migration required
+- [ ] Environment variables added/changed
+- [ ] Configuration changes needed
+
+## Additional Context
+<!-- Any other information -->

.github/workflows/lint.yml

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+name: Code Quality
+
+on:
+  push:
+    branches: [ main, develop ]
+  pull_request:
+    branches: [ main, develop ]
+
+jobs:
+  lint:
+    runs-on: ubuntu-latest
+    
+    steps:
+    - uses: actions/checkout@v4
+    
+    - name: Set up Python
+      uses: actions/setup-python@v5
+      with:
+        python-version: '3.11'
+    
+    - name: Install dependencies
+      run: |
+        python -m pip install --upgrade pip
+        pip install black ruff mypy
+        pip install -r backend/requirements.txt
+    
+    - name: Check code formatting with Black
+      run: |
+        black --check backend/
+    
+    - name: Lint with Ruff
+      run: |
+        ruff check backend/
+    
+    - name: Type check with mypy
+      run: |
+        mypy backend/app --ignore-missing-imports
+      continue-on-error: true

.github/workflows/security.yml

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+name: Security Scan
+
+on:
+  push:
+    branches: [ main, develop ]
+  pull_request:
+    branches: [ main, develop ]
+  schedule:
+    - cron: '0 0 * * 0'  # Weekly on Sunday
+
+jobs:
+  security:
+    runs-on: ubuntu-latest
+    
+    steps:
+    - uses: actions/checkout@v4
+    
+    - name: Set up Python
+      uses: actions/setup-python@v5
+      with:
+        python-version: '3.11'
+    
+    - name: Install dependencies
+      run: |
+        python -m pip install --upgrade pip
+        pip install bandit safety
+        pip install -r backend/requirements.txt
+    
+    - name: Run Bandit security scan
+      run: |
+        bandit -r backend/app -ll
+      continue-on-error: true
+    
+    - name: Check dependencies for known vulnerabilities
+      run: |
+        safety check --json
+      continue-on-error: true
+    
+    - name: Run CodeQL Analysis
+      uses: github/codeql-action/init@v3
+      with:
+        languages: python
+    
+    - name: Perform CodeQL Analysis
+      uses: github/codeql-action/analyze@v3

.github/workflows/test.yml

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+name: Tests
+
+on:
+  push:
+    branches: [ main, develop ]
+  pull_request:
+    branches: [ main, develop ]
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    
+    services:
+      postgres:
+        image: postgres:15
+        env:
+          POSTGRES_USER: postgres
+          POSTGRES_PASSWORD: postgres
+          POSTGRES_DB: pop3_forwarder_test
+        options: >-
+          --health-cmd pg_isready
+          --health-interval 10s
+          --health-timeout 5s
+          --health-retries 5
+        ports:
+          - 5432:5432
+      
+      redis:
+        image: redis:7-alpine
+        options: >-
+          --health-cmd "redis-cli ping"
+          --health-interval 10s
+          --health-timeout 5s
+          --health-retries 5
+        ports:
+          - 6379:6379
+    
+    steps:
+    - uses: actions/checkout@v4
+    
+    - name: Set up Python
+      uses: actions/setup-python@v5
+      with:
+        python-version: '3.11'
+        cache: 'pip'
+    
+    - name: Install dependencies
+      run: |
+        python -m pip install --upgrade pip
+        pip install -r backend/requirements.txt
+        pip install pytest pytest-asyncio pytest-cov httpx
+    
+    - name: Run tests with coverage
+      env:
+        DATABASE_URL: postgresql+asyncpg://postgres:postgres@localhost:5432/pop3_forwarder_test
+        REDIS_URL: redis://localhost:6379/0
+        SECRET_KEY: test-secret-key-for-ci-cd-at-least-32-chars
+        ENCRYPTION_KEY: test-encryption-key-for-ci-cd-at-least-32-chars
+      run: |
+        cd backend
+        pytest tests/ -v --cov=app --cov-report=xml --cov-report=term
+    
+    - name: Upload coverage to Codecov
+      uses: codecov/codecov-action@v3
+      with:
+        file: ./backend/coverage.xml
+        flags: unittests
+        name: codecov-umbrella
+        fail_ci_if_error: false