feat(sources): YouTube Music search + download pipeline (§1C/§1E)

Pluggable fetch source: ytmusicapi search + yt-dlp download (cookies-file guard), DownloadJob entity/repo + DownloadService, download_task worker with exponential-backoff retries, and wired /search, /sources/{source}/search, and /downloads endpoints. Adds youtube_enabled/cookies config, yt-dlp+ytmusicapi deps, and the download_jobs.track_id migration. Snapshot also bundles in-progress storage/tracks/acoustid edits.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/api/schemas/download.py

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+"""Schemas for the download job endpoints (§A5 download manager)."""
+
+import datetime as dt
+import uuid
+
+from pydantic import BaseModel, Field
+
+from app.domain.entities.download import DownloadJob
+
+
+class DownloadCreate(BaseModel):
+    """Request to download an item discovered on a fetch source."""
+
+    source: str
+    source_id: str = Field(min_length=1)
+    # Optional free-text the result came from — stored for display only.
+    query: str | None = None
+
+
+class DownloadJobOut(BaseModel):
+    id: uuid.UUID
+    source: str
+    source_id: str | None
+    query: str | None
+    status: str
+    progress: float
+    error_message: str | None
+    retry_count: int
+    track_id: uuid.UUID | None
+    created_at: dt.datetime
+    updated_at: dt.datetime
+
+    @classmethod
+    def from_entity(cls, job: DownloadJob) -> DownloadJobOut:
+        return cls(
+            id=job.id,
+            source=job.source,
+            source_id=job.source_id,
+            query=job.query,
+            status=job.status,
+            progress=job.progress,
+            error_message=job.error_message,
+            retry_count=job.retry_count,
+            track_id=job.track_id,
+            created_at=job.created_at,
+            updated_at=job.updated_at,
+        )
+
+
+class DownloadCreateResponse(BaseModel):
+    """Result of requesting a download.
+
+    ``already_in_library`` → the item was already imported (``track_id`` set, no
+    job). Otherwise ``job`` describes the queued (or already in-flight) download.
+    """
+
+    already_in_library: bool
+    track_id: uuid.UUID | None
+    job: DownloadJobOut | None

app/api/schemas/external_search.py

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+"""Schemas for searching external (fetch) sources — the §A4 discover screen."""
+
+from pydantic import BaseModel
+
+from app.domain.sources import SearchResult
+
+
+class ExternalSearchResultOut(BaseModel):
+    source: str
+    source_id: str
+    title: str
+    artist: str | None
+    album: str | None
+    duration_seconds: int | None
+    thumbnail_url: str | None
+
+    @classmethod
+    def from_entity(cls, r: SearchResult) -> ExternalSearchResultOut:
+        return cls(
+            source=r.source,
+            source_id=r.source_id,
+            title=r.title,
+            artist=r.artist,
+            album=r.album,
+            duration_seconds=r.duration_seconds,
+            thumbnail_url=r.thumbnail_url,
+        )
+
+
+class ExternalSearchResponse(BaseModel):
+    """Flat list of hits across one or more searchable sources, plus the names of
+    sources that were unavailable (so the UI can show a soft warning)."""
+
+    results: list[ExternalSearchResultOut]
+    searched_sources: list[str]