feat(config): derive MusicBrainz/AcoustID User-Agent from app name+version

Replace the placeholder MUSICBRAINZ_USER_AGENT env var with
MUSICBRAINZ_OWNER_EMAIL. The User-Agent ("MCMA/<version> ( <contact> )")
is now composed from the fixed app name, the installed package version,
and the operator's contact email — falling back to the project URL when
no email is configured. Also use the same version for the FastAPI app.

app/main.py

@@ -10,7 +10,7 @@ from app.api.health import router as health_router
 from app.api.middleware import CorrelationIdMiddleware
 from app.api.rest import subsonic_router
 from app.api.v1 import api_v1_router
-from app.core.config import get_settings
+from app.core.config import app_version, get_settings
 from app.core.logging import configure_logging, get_logger
 from app.infrastructure.cache import close_redis
 from app.infrastructure.db import dispose_engine
@@ -34,7 +34,7 @@ def create_app() -> FastAPI:
 
     app = FastAPI(
         title="mcma-backend",
-        version="0.1.0",
+        version=app_version(),
         summary="Self-hosted, offline-first music service.",
         lifespan=lifespan,
     )