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/v1/sources.py

@@ -1,14 +1,13 @@
-"""External source endpoints: enumerate sources and trigger imports.
+"""External source endpoints: enumerate sources, search, and trigger imports.
 
-Listing/health are read-only (any authenticated user). Scanning a source is an
-admin action and runs in a worker — the endpoint only enqueues it.
+Listing/health/search are read-only (any authenticated user). Scanning a source
+is an admin action and runs in a worker — the endpoint only enqueues it.
 """
 
-from typing import Any
-
-from fastapi import APIRouter
+from fastapi import APIRouter, Query
 
 from app.api.deps import CurrentUser, SourceRegistryDep, SuperUser
+from app.api.schemas.external_search import ExternalSearchResponse, ExternalSearchResultOut
 from app.api.schemas.source import ScanResponse, SourceHealthOut, SourceInfoOut
 from app.domain.errors import DependencyUnavailableError
 from app.workers.queue import enqueue
@@ -39,6 +38,18 @@ async def source_health(
 
 
 @router.get("/{source}/search")
-async def search_source(source: str, _: CurrentUser) -> Any:
-    # Search is for fetch-style sources (youtube, …) — not yet implemented.
-    ...
+async def search_source(
+    source: str,
+    _: CurrentUser,
+    registry: SourceRegistryDep,
+    q: str = Query(min_length=1),
+    limit: int = Query(20, ge=1, le=50),
+) -> ExternalSearchResponse:
+    backend = registry.searchable(source)  # 404 if unknown, 422 if not searchable
+    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],
+    )