Phase 7: Hardening — logging, security, Docker, production readiness

Backend:
- Structured JSON logging (python-json-logger) with request ID correlation
- RequestIDMiddleware (server-generated UUID, no client trust)
- Global exception handlers: AppException, RequestValidationError, generic 500
  — all return consistent {"error": {code, message, request_id}} format
- Async rate limiting with lock + stale key eviction on auth endpoints
- Health endpoint checks DB connectivity, returns version + status
- Custom exception classes (NotFoundException, ForbiddenException, etc.)
- OpenAPI docs with tag descriptions, conditional URL (disabled in production)
- LOG_LEVEL, DOCS_ENABLED, RATE_LIMIT_* settings added

Docker:
- Backend: multi-stage build (builder + runtime), non-root user, HEALTHCHECK
- Frontend: removed dead user, HEALTHCHECK directive
- docker-compose: restart policies, healthchecks, Redis service, named volumes
  for uploads/PDFs, rate limit env vars forwarded
- Alembic migrations run only in Dockerfile CMD (removed from lifespan)

Nginx:
- server_tokens off
- CSP, Referrer-Policy, Permissions-Policy headers
- HSTS ready (commented, enable with TLS)

Config & Docs:
- .env.production.example with production-ready settings
- CLAUDE.md project conventions (structure, workflow, naming, how-to)
- .env.example updated with new variables

Review fixes applied:
- Rate limiter: async lock prevents race condition, stale key eviction
- Request ID: always server-generated (no log injection)
- Removed duplicate alembic migration from lifespan
- Removed dead app user from frontend Dockerfile
- Health check logs DB errors
- Rate limit env vars forwarded in docker-compose

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

nginx/nginx.conf

@@ -1,3 +1,5 @@
+server_tokens off;
+
 upstream backend {
     server backend:8000;
 }
@@ -20,6 +22,11 @@ server {
     add_header X-Frame-Options "SAMEORIGIN" always;
     add_header X-Content-Type-Options "nosniff" always;
     add_header X-XSS-Protection "1; mode=block" always;
+    add_header Referrer-Policy "strict-origin-when-cross-origin" always;
+    add_header Permissions-Policy "camera=(), microphone=(), geolocation=()" always;
+    add_header Content-Security-Policy "default-src 'self'; script-src 'self'; style-src 'self' 'unsafe-inline'; img-src 'self' data:; connect-src 'self' ws: wss:;" always;
+    # Enable with TLS:
+    # add_header Strict-Transport-Security "max-age=31536000; includeSubDomains" always;
 
     # API (with SSE support)
     location /api/ {