feat: Milestone 1 Security & Infrastructure improvements

- Add Dependabot configuration for pip, npm, GitHub Actions, Docker
- Update copilot instructions to require TODO.md and CHANGELOG.md updates
- Add unit tests for middleware, security, app factory, and schemas (125 total tests, 57% coverage)
- Update TODO.md with current progress
- Update CHANGELOG.md with all changes

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>
Agent-Logs-Url: https://github.com/christianlouis/pop_puller_to_gmail/sessions/2f15a52f-6812-4586-9d5c-a144226c842b

.github/copilot-instructions.md

@@ -24,6 +24,13 @@ cd frontend
 npm run lint
 ```
 
+## Documentation Requirements
+
+When making changes, always update the following files:
+
+- **`docs/TODO.md`**: Update task checkboxes, progress percentages, and status indicators to reflect completed work and any new items discovered.
+- **`CHANGELOG.md`**: Add entries under the `[Unreleased]` section using the appropriate category (`Added`, `Changed`, `Fixed`, `Security`, `Removed`, `Deprecated`).
+
 ## Conventions
 
 - **Python**: Follow PEP 8. Use `black` for formatting. All ruff and mypy errors must be resolved before committing.

.github/dependabot.yml

@@ -0,0 +1,53 @@
+version: 2
+updates:
+  # Python dependencies (backend)
+  - package-ecosystem: "pip"
+    directory: "/backend"
+    schedule:
+      interval: "weekly"
+      day: "monday"
+    open-pull-requests-limit: 10
+    labels:
+      - "dependencies"
+      - "python"
+    commit-message:
+      prefix: "chore(deps):"
+
+  # GitHub Actions
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+      day: "monday"
+    open-pull-requests-limit: 5
+    labels:
+      - "dependencies"
+      - "github-actions"
+    commit-message:
+      prefix: "chore(ci):"
+
+  # npm dependencies (frontend)
+  - package-ecosystem: "npm"
+    directory: "/frontend"
+    schedule:
+      interval: "weekly"
+      day: "monday"
+    open-pull-requests-limit: 10
+    labels:
+      - "dependencies"
+      - "javascript"
+    commit-message:
+      prefix: "chore(deps):"
+
+  # Docker dependencies
+  - package-ecosystem: "docker"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+      day: "monday"
+    open-pull-requests-limit: 5
+    labels:
+      - "dependencies"
+      - "docker"
+    commit-message:
+      prefix: "chore(docker):"