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>

backend/app/core/rate_limit.py

@@ -0,0 +1,39 @@
+"""In-memory sliding window rate limiter.
+
+Note: For multi-instance deployments, swap to Redis-backed implementation.
+"""
+import asyncio
+import time
+from collections import defaultdict
+
+from fastapi import Request, HTTPException, status
+
+from app.config import settings
+
+_requests: dict[str, list[float]] = defaultdict(list)
+_lock = asyncio.Lock()
+
+
+async def check_rate_limit(request: Request) -> None:
+    """Check if the request IP is within rate limits. Raises 429 if exceeded."""
+    client_ip = request.client.host if request.client else "unknown"
+    now = time.time()
+    window = settings.RATE_LIMIT_WINDOW_SECONDS
+    max_requests = settings.RATE_LIMIT_REQUESTS
+
+    async with _lock:
+        # Clean old entries
+        _requests[client_ip] = [t for t in _requests[client_ip] if t > now - window]
+
+        if len(_requests[client_ip]) >= max_requests:
+            raise HTTPException(
+                status_code=status.HTTP_429_TOO_MANY_REQUESTS,
+                detail="Too many requests. Please try again later.",
+            )
+
+        _requests[client_ip].append(now)
+
+        # Evict empty keys to prevent unbounded growth
+        stale = [ip for ip, ts in _requests.items() if not ts]
+        for ip in stale:
+            del _requests[ip]