feat: implement audit logging middleware for sensitive API operations

- Add AuditMiddleware that logs POST/PUT/PATCH/DELETE and GET /auth/me
- Logs structured JSON: event, timestamp, user_id, method, path, client_ip, status_code, duration_ms
- Excludes health endpoints and OPTIONS requests
- Never logs request/response bodies or auth headers/cookies
- Wire user_id from auth dependency via request.state
- Add add_audit_middleware() to app factory

Co-Authored-By: Paperclip <noreply@paperclip.ing>

src/cartsnitch_api/middleware/audit.py

@@ -0,0 +1,64 @@
+"""Audit logging middleware for sensitive API operations.
+
+Logs structured JSON for POST/PUT/PATCH/DELETE requests and GET /auth/me.
+Never logs request bodies, response bodies, Authorization headers, or cookie values.
+"""
+
+import json
+import logging
+import time
+from collections.abc import Awaitable, Callable
+
+from fastapi import FastAPI, Request
+from starlette.middleware.base import BaseHTTPMiddleware
+
+logger = logging.getLogger("cartsnitch_api.audit")
+
+HEALTH_PATHS = {"/health", "/healthz", "/ready"}
+
+
+class AuditMiddleware(BaseHTTPMiddleware):
+    """Middleware to log structured audit events for sensitive operations."""
+
+    async def dispatch(
+        self,
+        request: Request,
+        call_next: Callable[[Request], Awaitable],
+    ):
+        if request.method == "OPTIONS" or request.url.path in HEALTH_PATHS:
+            return await call_next(request)
+
+        method = request.method
+        path = request.url.path
+
+        is_sensitive_write = method in {"POST", "PUT", "PATCH", "DELETE"}
+        is_auth_me_read = method == "GET" and path == "/auth/me"
+
+        if not (is_sensitive_write or is_auth_me_read):
+            return await call_next(request)
+
+        start = time.perf_counter()
+        response = await call_next(request)
+        duration_ms = (time.perf_counter() - start) * 1000
+
+        user_id = getattr(request.state, "user_id", None)
+        client_ip = request.client.host if request.client else "unknown"
+
+        log_entry = {
+            "event": "audit",
+            "timestamp": time.strftime("%Y-%m-%dT%H:%M:%SZ", time.gmtime()),
+            "user_id": user_id,
+            "method": method,
+            "path": path,
+            "client_ip": client_ip,
+            "status_code": response.status_code,
+            "duration_ms": round(duration_ms, 2),
+        }
+
+        logger.info(json.dumps(log_entry))
+
+        return response
+
+
+def add_audit_middleware(app: FastAPI) -> None:
+    app.add_middleware(AuditMiddleware)