feat(hardening): login rate limiting, per-service memory limits, pinned images

- slowapi limiter on POST /auth/login (10/min per IP, Persian error msg);
  edge-level coarse limits stay in Traefik
- mem_limit on every main-compose service (~4.4GB worst case, protects the
  8GB shared host during rebuilds)
- monitoring images pinned to verified tags (prom v3.4.1, grafana 11.6.3,
  loki/promtail 3.5.0, node-exporter v1.9.1, postgres-exporter v0.17.1,
  cadvisor v0.49.1); exporter image name fixed to postgres-exporter

backend/app/routers/auth.py

@@ -13,7 +13,7 @@ Error mapping (documented in docs/casdoor-auth.md):
 import logging
 
 import httpx
-from fastapi import APIRouter, Depends
+from fastapi import APIRouter, Depends, Request
 from pydantic import BaseModel
 from sqlalchemy.ext.asyncio import AsyncSession
 
@@ -23,6 +23,7 @@ from app.db.engine import get_db
 from app.routers.users import _user_projects, user_to_dict
 from app.services import analytics
 from app.utils.errors import ServiceUnavailableError, UnauthorizedError
+from app.utils.ratelimit import limiter
 
 logger = logging.getLogger("procurement.auth")
 
@@ -35,7 +36,8 @@ class LoginRequest(BaseModel):
 
 
 @router.post("/login")
-async def login(payload: LoginRequest, db: AsyncSession = Depends(get_db)):
+@limiter.limit("10/minute", error_message="تلاش‌های ورود بیش از حد مجاز — کمی بعد دوباره امتحان کنید")
+async def login(request: Request, payload: LoginRequest, db: AsyncSession = Depends(get_db)):
     """Exchange username/password for a Casdoor JWT (OAuth password grant)."""
     url = f"{settings.CASDOOR_ENDPOINT}/api/login/oauth/access_token"
     form = {