Complete Phase 7: Polish & Cross-Cutting Concerns

This commit implements all remaining polish tasks (T193-T210) to make
the application production-ready.

## Logging & Monitoring (T193, T194, T208, T209)
- Add structured JSON logging for production environments
- Add human-readable logging for development
- Implement comprehensive error logging across all routes:
  * submission.py: product access, validation, success/failure
  * auth.py: login attempts, successes, failures, logouts
  * dashboard.py: access and errors
- Add /health endpoint for monitoring (checks data dir, API key)
- Add environment variable validation on startup

## Security Hardening (T196-T199, T207)
- Add HSTS headers in production (1 year, includeSubDomains)
- Add security headers: X-Content-Type-Options, X-Frame-Options, X-XSS-Protection
- Verify CSRF protection on all POST routes (Flask-WTF)
- Verify session cookie security flags (HttpOnly, Secure, SameSite)
- Verify XSS prevention (Jinja2 auto-escaping)
- Verify no hardcoded secrets (only in test files)

## Documentation (T195, T203, T210)
- Add comprehensive README.md with:
  * Features, quick start, project structure
  * Usage guides (end users, product owners, admins)
  * Configuration, testing, deployment instructions
- Add detailed docs/deployment.md with:
  * Production deployment steps
  * ClamAV, Nginx, SSL/TLS setup
  * Security hardening, monitoring, backup strategies
- Add requirements-dev.txt for development dependencies

## Performance Testing (T200, T201)
- Add test_performance.py with 4 comprehensive tests:
  * 100 concurrent submissions (SC-012)
  * Dashboard load <3s for 1000 items (SC-008)
  * Large file upload handling
  * Rate limiting verification
- Add performance marker to pytest.ini

## Testing
- All 49 tests passing, 1 skipped
- Fixed error handling to preserve HTTP status codes

Phase 7 complete. Application is production-ready with comprehensive
logging, security, monitoring, and documentation.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

pytest.ini

@@ -11,3 +11,4 @@ markers =
     contract: Contract tests for API endpoints
     integration: Integration tests for user journeys
     unit: Unit tests for isolated components
+    performance: Performance tests for scalability and load testing