Add X-User-ID header extraction to FastAPI service.

Create a get_user_id() dependency that extracts the X-User-ID header
from incoming requests, making it available to route handlers.
The dependency is optional (not enforced) — callers decide whether
to use it or require it on specific routes.

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

services/ml/app/main.py

@@ -58,6 +58,18 @@ async def verify_api_key(x_api_key: str = Header(default="")):
         raise HTTPException(status_code=401, detail="Unauthorized")
 
 
+# --- User ID Header Dependency ---
+
+async def get_user_id(x_user_id: str = Header(default="")) -> Optional[str]:
+    """
+    Extract X-User-ID header from incoming requests.
+
+    Returns the user ID if present, or None if not provided.
+    This is optional — callers decide whether to enforce it.
+    """
+    return x_user_id if x_user_id else None
+
+
 # --- Lifespan ---
 
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