feat(06-05): trusted-proxy get_client_ip, per-account rate limiter, promote 8 xfail tests (D-11/D-12/D-13)

- D-11: replace get_client_ip body with trusted-proxy CIDR check (127/8, 172.16/12,
  192.168/16, ::1/128); untrusted peers always return their own IP, preventing XFF
  spoofing by external callers
- D-12: create services/rate_limiting.py with _account_key() keyed by user.id
  (falls back to peer IP when no authenticated user in request.state); exports
  account_limiter = Limiter(key_func=_account_key)
- Wire: auth.py switches from get_remote_address to get_client_ip; main.py imports
  account_limiter; all 9 documents.py endpoints and 7 cloud.py endpoints decorated
  @account_limiter.limit("100/minute") with request.state.current_user = current_user
  as the first handler statement (A1 ordering invariant)
- Promote all 8 xfail stubs to real assertions; add autouse fixture in conftest.py
  to reset MemoryStorage between tests preventing cross-test 429 contamination

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/services/rate_limiting.py

@@ -0,0 +1,17 @@
+"""Per-account rate limiter shared across document and cloud routers (D-12)."""
+from __future__ import annotations
+
+from fastapi import Request
+from slowapi import Limiter
+
+
+def _account_key(request: Request) -> str:
+    user = getattr(request.state, "current_user", None)
+    if user is not None:
+        return str(user.id)
+    if request.client:
+        return request.client.host
+    return "anonymous"
+
+
+account_limiter = Limiter(key_func=_account_key)