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/schemas/external_search.py

@@ -1,7 +1,10 @@
 """Schemas for searching external (fetch) sources — the §A4 discover screen."""
 
+import uuid
+
 from pydantic import BaseModel
 
+from app.domain.entities.track import Track
 from app.domain.sources import SearchResult
 
 
@@ -13,9 +16,16 @@ class ExternalSearchResultOut(BaseModel):
     album: str | None
     duration_seconds: int | None
     thumbnail_url: str | None
+    # Remote browse (plan: Model C) — set when this hit is already saved in the
+    # library, so the UI can show "Play"/"Saved" instead of "Save to library".
+    in_library: bool
+    track_id: uuid.UUID | None
+    availability: str | None
 
     @classmethod
-    def from_entity(cls, r: SearchResult) -> ExternalSearchResultOut:
+    def from_entity(
+        cls, r: SearchResult, *, existing: Track | None = None
+    ) -> ExternalSearchResultOut:
         return cls(
             source=r.source,
             source_id=r.source_id,
@@ -24,6 +34,9 @@ class ExternalSearchResultOut(BaseModel):
             album=r.album,
             duration_seconds=r.duration_seconds,
             thumbnail_url=r.thumbnail_url,
+            in_library=existing is not None,
+            track_id=existing.id if existing is not None else None,
+            availability=existing.availability if existing is not None else None,
         )