fix: login endpoint - request variable shadowing Starlette Request

The login() function used 'request: LoginRequest' which shadowed
FastAPI's Starlette Request object. Request.client was accessed on
the LoginRequest Pydantic model instead of the HTTP request, causing
AttributeError: 'LoginRequest' object has no attribute 'client'.

Fix: rename the Pydantic parameter to 'body' and add explicit
'request: Request' for IP extraction and rate limiting.

backend/auth/router.py

@@ -89,14 +89,14 @@ async def auth_status():
 
 
 @router.post("/login")
-async def login(request: LoginRequest, response: Response):
+async def login(body: LoginRequest, response: Response, request: Request):
     """Authenticate a user. Returns access token and sets refresh cookie.
 
     Implements timing-safe responses to prevent user enumeration:
     a failed login with an unknown user takes the same time as one
     with a known user (dummy hash is computed).
     """
-    user = get_user(request.username)
+    user = get_user(body.username)
 
     if not user:
         # Timing-safe: simulate hash computation to prevent user enumeration
@@ -111,11 +111,11 @@ async def login(request: LoginRequest, response: Response):
     if is_rate_limited(client_ip):
         raise HTTPException(429, "Trop de tentatives depuis cette adresse IP (15min)")
 
-    if is_locked(request.username):
+    if is_locked(body.username):
         raise HTTPException(429, "Compte temporairement verrouillé (15min)")
 
-    if not verify_password(request.password, user["password_hash"]):
-        attempts = record_login_failure(request.username)
+    if not verify_password(body.password, user["password_hash"]):
+        attempts = record_login_failure(body.username)
         rl_attempts, rl_remaining = rl_record_failure(client_ip)
         remaining = max(0, 5 - attempts)
         detail = "Identifiants invalides"
@@ -124,14 +124,14 @@ async def login(request: LoginRequest, response: Response):
         raise HTTPException(401, detail)
 
     # Success — clear rate limits and generate tokens
-    record_login_success(request.username)
+    record_login_success(body.username)
     rl_record_success(client_ip)
 
     access_token = create_access_token(user)
-    refresh_token, refresh_jti = create_refresh_token(request.username)
+    refresh_token, refresh_jti = create_refresh_token(body.username)
 
     # Set refresh token as HttpOnly cookie (path-restricted to /api/auth/refresh)
-    max_age = 2592000 if request.remember_me else 604800  # 30d or 7d
+    max_age = 2592000 if body.remember_me else 604800  # 30d or 7d
     import os
     secure = os.environ.get("OBSIGATE_SECURE_COOKIES", "false").lower() == "true"
     response.set_cookie(
@@ -144,7 +144,7 @@ async def login(request: LoginRequest, response: Response):
         path="/api/auth/refresh",
     )
 
-    logger.info(f"User '{request.username}' logged in")
+    logger.info(f"User '{body.username}' logged in")
 
     # Set access token as cookie for same-origin requests (e.g. popout window)
     response.set_cookie(