security: implement Quick Wins security middleware (inst_041-046)
- Add security headers middleware (CSP, HSTS, X-Frame-Options, etc.)
- Add rate limiting (100 req/15min public, 5 req/min forms)
- Add input validation and sanitization middleware
- Add response sanitization (hide stack traces, remove sensitive fields)
- Add centralized security event logging to audit trail
- Disable CSRF (deprecated package, will implement modern solution in Phase 3)
- Update security logger to use HOME-based log path
Implements: inst_041, inst_042, inst_043, inst_044, inst_045, inst_046
Refs: docs/plans/security-implementation-roadmap.md
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