feat: API key authentication dependency

Implements create_api_key_dependency() FastAPI dependency that validates
Bearer tokens against a configured list of ApiKey objects (401 on missing,
malformed, or unknown tokens). Includes 5 TDD tests covering all cases.

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

kischdle/llmux/llmux/auth.py

@@ -0,0 +1,19 @@
+from fastapi import HTTPException, Request
+
+from llmux.config import ApiKey
+
+
+def create_api_key_dependency(api_keys: list[ApiKey]):
+    key_to_name = {k.key: k.name for k in api_keys}
+
+    async def require_api_key(request: Request) -> str:
+        auth = request.headers.get("Authorization", "")
+        if not auth.startswith("Bearer "):
+            raise HTTPException(status_code=401, detail="Missing or malformed Authorization header")
+        token = auth[7:]
+        name = key_to_name.get(token)
+        if name is None:
+            raise HTTPException(status_code=401, detail="Invalid API key")
+        return name
+
+    return require_api_key