feat(security): merge Phase 4b security hardening

Complete security hardening implementation including HTTPS enforcement,
security headers, rate limiting, and comprehensive security test suite.

Key features:
- HTTPS enforcement with HSTS support
- Security headers (CSP, X-Frame-Options, X-Content-Type-Options)
- Rate limiting for all critical endpoints
- Enhanced email template security
- 87% test coverage with security-specific tests

Architect approval: 9.5/10

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

pyproject.toml

@@ -111,6 +111,8 @@ markers = [
     "unit: Unit tests",
     "integration: Integration tests",
     "e2e: End-to-end tests",
+    "security: Security-related tests (timing attacks, injection, headers)",
+    "slow: Tests that take longer to run (timing attack statistics)",
 ]
 
 [tool.coverage.run]