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/track.py

@@ -3,7 +3,9 @@
 import datetime as dt
 import uuid
 
-from pydantic import BaseModel
+from pydantic import BaseModel, Field
+
+from app.api.schemas.download import DownloadJobOut
 
 
 class TrackOut(BaseModel):
@@ -62,3 +64,24 @@ class MetadataApply(BaseModel):
     year: int | None = None
     genre: str | None = None
     track_number: int | None = None
+
+
+class RemoteTrackSave(BaseModel):
+    """Save a remote browse hit (§A4 discover) as a library placeholder —
+    ``availability="remote"``, no audio until first play (plan: Model C)."""
+
+    source: str
+    source_id: str = Field(min_length=1)
+    title: str
+    artist: str | None = None
+
+
+class MaterializeResponse(BaseModel):
+    """Result of requesting that a placeholder track's audio be fetched.
+
+    ``job`` is ``None`` when the track is already ``local`` — nothing to wait
+    for, the caller can stream immediately. Otherwise it's the (new or
+    already in-flight) job; poll ``GET /downloads/{job.id}`` until ``done``."""
+
+    track: TrackOut
+    job: DownloadJobOut | None