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 -->