feat(api): Add /users/usage endpoint (#96)

- Create routes/users.py with usage and limit check endpoints
- GET /users/usage - returns tier, repo count, limits
- GET /users/limits/check-repo-add - pre-check before adding repo
- Register users_router in main.py

Author: DevanshuNEU

backend/main.py

@@ -25,6 +25,7 @@
 from routes.search import router as search_router
 from routes.analysis import router as analysis_router
 from routes.api_keys import router as api_keys_router
+from routes.users import router as users_router
 
 
 # Lifespan context manager for startup/shutdown
@@ -86,6 +87,7 @@ async def dispatch(self, request: Request, call_next):
 app.include_router(search_router, prefix=API_PREFIX)
 app.include_router(analysis_router, prefix=API_PREFIX)
 app.include_router(api_keys_router, prefix=API_PREFIX)
+app.include_router(users_router, prefix=API_PREFIX)
 
 # WebSocket endpoint (versioned)
 app.add_api_websocket_route(f"{API_PREFIX}/ws/index/{{repo_id}}", websocket_index)

backend/routes/users.py

@@ -0,0 +1,65 @@
+"""User routes - profile and usage information."""
+from fastapi import APIRouter, Depends
+
+from dependencies import user_limits
+from middleware.auth import require_auth, AuthContext
+from services.observability import logger
+
+router = APIRouter(prefix="/users", tags=["Users"])
+
+
+@router.get("/usage")
+async def get_user_usage(auth: AuthContext = Depends(require_auth)):
+    """
+    Get current user's usage and limits.
+    
+    Returns:
+        - tier: Current subscription tier
+        - repositories: Current count and limit
+        - limits: All tier limits
+        - features: Available features for tier
+    """
+    user_id = auth.user_id
+    
+    if not user_id:
+        logger.warning("Usage check without user_id", identifier=auth.identifier)
+        # Return free tier defaults for API key users
+        return {
+            "tier": "free",
+            "repositories": {
+                "current": 0,
+                "limit": 3,
+                "display": "0/3"
+            },
+            "limits": {
+                "max_files_per_repo": 500,
+                "max_functions_per_repo": 2000,
+                "playground_searches_per_day": 50,
+            },
+            "features": {
+                "priority_indexing": False,
+                "mcp_access": True,
+            }
+        }
+    
+    usage = await user_limits.get_usage_summary(user_id)
+    logger.info("Usage retrieved", user_id=user_id, tier=usage.get("tier"))
+    
+    return usage
+
+
+@router.get("/limits/check-repo-add")
+async def check_can_add_repo(auth: AuthContext = Depends(require_auth)):
+    """
+    Check if user can add another repository.
+    
+    Call this before showing "Add Repository" button to
+    disable it if limit reached.
+    """
+    user_id = auth.user_id
+    
+    if not user_id:
+        return {"allowed": True, "message": "OK"}
+    
+    result = await user_limits.check_repo_count(user_id)
+    return result.to_dict()