feat(server): FastAPI app with API-key auth and health check

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

server/app.py

@@ -0,0 +1,37 @@
+"""MusicFetch REST API. Plain HTTP behind an upstream TLS reverse proxy."""
+import os
+
+from fastapi import Depends, FastAPI, Header, HTTPException
+from fastapi.responses import JSONResponse
+
+from . import actions, jobs, mf
+
+API_KEY = os.environ.get("MUSICFETCH_API_KEY", "")
+ROOT = os.environ.get("MUSICFETCH_ROOT", "/media/music")
+
+app = FastAPI(title="MusicFetch API")
+
+
+def require_key(x_api_key: str = Header(default="")):
+    if not API_KEY or x_api_key != API_KEY:
+        raise HTTPException(status_code=401, detail="Invalid API key.")
+
+
+@app.exception_handler(HTTPException)
+async def _http_exc(_req, exc: HTTPException):
+    # Always return a Siri-speakable {"message": ...} body.
+    return JSONResponse(status_code=exc.status_code, content={"message": exc.detail})
+
+
+@app.get("/health")
+def health():
+    return {"status": "ok"}
+
+
+# Minimal stub so that auth tests can verify 401 behaviour before the real
+# /fetch implementation lands in Task 5. This route MUST carry require_key as
+# a dependency so unauthenticated / wrong-key requests are rejected here.
+@app.post("/fetch", dependencies=[Depends(require_key)])
+def fetch_stub(q: str = ""):
+    # Task 5 will replace this body with the full search-and-download logic.
+    raise HTTPException(status_code=501, detail="Not yet implemented.")