feat(library): remote browse status + save/materialize API (§Phase2-3)

Search results now report whether a hit is already saved (in_library,
track_id, availability). New RemoteLibraryService backs POST
/tracks/remote (idempotent placeholder save) and POST
/tracks/{id}/materialize (on-demand fetch via a new materialize_track
arq task, reusing in-flight jobs).

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

app/api/v1/sources.py

@@ -6,7 +6,7 @@ is an admin action and runs in a worker — the endpoint only enqueues it.
 
 from fastapi import APIRouter, Query
 
-from app.api.deps import CurrentUser, SourceRegistryDep, SuperUser
+from app.api.deps import CurrentUser, SourceRegistryDep, SuperUser, TrackRepoDep
 from app.api.schemas.external_search import ExternalSearchResponse, ExternalSearchResultOut
 from app.api.schemas.source import ScanResponse, SourceHealthOut, SourceInfoOut
 from app.domain.errors import DependencyUnavailableError
@@ -42,6 +42,7 @@ async def search_source(
     source: str,
     _: CurrentUser,
     registry: SourceRegistryDep,
+    track_repo: TrackRepoDep,
     q: str = Query(min_length=1),
     limit: int = Query(20, ge=1, le=50),
 ) -> ExternalSearchResponse:
@@ -49,7 +50,8 @@ async def search_source(
     if not backend.is_available():
         raise DependencyUnavailableError(f"Source {source!r} is not available.")
     results = await backend.search(q, limit=limit)
-    return ExternalSearchResponse(
-        results=[ExternalSearchResultOut.from_entity(r) for r in results],
-        searched_sources=[source],
-    )
+    out: list[ExternalSearchResultOut] = []
+    for r in results:
+        existing = await track_repo.get_by_source(r.source, r.source_id)
+        out.append(ExternalSearchResultOut.from_entity(r, existing=existing))
+    return ExternalSearchResponse(results=out, searched_sources=[source])