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/domain/ports.py

@@ -7,7 +7,7 @@ are bound to these ports at the composition root (``app.api.deps``).
 
 import datetime as dt
 import uuid
-from collections.abc import AsyncIterator, Iterator
+from collections.abc import AsyncIterator, Awaitable, Callable, Iterator
 from contextlib import AbstractAsyncContextManager
 from pathlib import Path
 from typing import Protocol
@@ -18,6 +18,7 @@ from app.domain.entities import (
     CoverArt,
     Credentials,
     DiskUsage,
+    DownloadJob,
     Fingerprint,
     LibraryStats,
     Like,
@@ -29,9 +30,14 @@ from app.domain.entities import (
     User,
 )
 from app.domain.entities.track import Artist, Track
-from app.domain.sources import SourceFile, SourceInfo
+from app.domain.sources import DownloadResult, RawMetadata, SearchResult, SourceFile, SourceInfo
 from app.domain.tokens import IssuedToken, TokenClaims, TokenType
 
+# A fetch source reports download progress as a fraction in [0.0, 1.0]. It's a
+# plain callback (not a port) because it's an inversion of control supplied per
+# call by the worker, which persists it to the download job.
+ProgressCallback = Callable[[float], Awaitable[None]]
+
 
 class UserRepository(Protocol):
     async def get_by_id(self, user_id: uuid.UUID) -> User | None: ...
@@ -275,6 +281,54 @@ class HistoryRepository(Protocol):
     async def count(self, *, user_id: uuid.UUID) -> int: ...
 
 
+class DownloadJobRepository(Protocol):
+    """Persistence for download jobs (plan §6.1). Drives the §A5 download manager
+    and the worker's retry/backoff loop."""
+
+    async def add(
+        self,
+        *,
+        source: str,
+        source_id: str | None,
+        query: str | None,
+        requested_by: uuid.UUID | None,
+    ) -> DownloadJob: ...
+    async def get_by_id(self, job_id: uuid.UUID) -> DownloadJob | None: ...
+    async def get_active_for_source(self, source: str, source_id: str) -> DownloadJob | None:
+        """An unfinished (queued/downloading/enriching) job for the same item, if
+        any — used to dedup before enqueuing so a double-click can't queue twice."""
+        ...
+
+    async def list(
+        self,
+        *,
+        requested_by: uuid.UUID | None,
+        status: str | None,
+        limit: int,
+        offset: int,
+    ) -> list[DownloadJob]: ...
+    async def count(self, *, requested_by: uuid.UUID | None, status: str | None) -> int: ...
+    async def set_status(
+        self,
+        job_id: uuid.UUID,
+        *,
+        status: str,
+        error_message: str | None = None,
+        track_id: uuid.UUID | None = None,
+    ) -> None: ...
+    async def set_progress(self, job_id: uuid.UUID, progress: float) -> None: ...
+    async def increment_retry(self, job_id: uuid.UUID) -> int:
+        """Bump ``retry_count`` and return the new value."""
+        ...
+
+    async def delete(self, job_id: uuid.UUID) -> None: ...
+    async def failure_rate(self, source: str, *, since: dt.datetime) -> float:
+        """Fraction of jobs for ``source`` created since ``since`` that ended
+        ``failed`` (0.0 when there are none) — drives the §A5 "source unhealthy"
+        banner."""
+        ...
+
+
 class SourceBackend(Protocol):
     """A registered source of tracks (mounted folder, YouTube, …).
 
@@ -293,6 +347,29 @@ class IndexableSource(SourceBackend, Protocol):
     def scan(self) -> Iterator[SourceFile]: ...
 
 
+class SearchableSource(SourceBackend, Protocol):
+    """A source that can be searched by free text (e.g. YouTube Music).
+
+    Returns ``[]`` (never raises) on no results / the service being down — the
+    discover screen degrades to "nothing found" rather than erroring."""
+
+    async def search(self, query: str, *, limit: int) -> list[SearchResult]: ...
+
+
+class FetchableSource(SourceBackend, Protocol):
+    """A source that can download a previously-discovered item to local disk.
+
+    ``fetch`` resolves a ``source_id`` (from a :class:`SearchResult`) into a file
+    and reports progress through ``on_progress``. It runs only in a worker (heavy
+    I/O) and raises on failure so the download task can retry with backoff."""
+
+    async def fetch(
+        self, source_id: str, *, on_progress: ProgressCallback | None = None
+    ) -> DownloadResult: ...
+
+    async def get_metadata(self, source_id: str) -> RawMetadata | None: ...
+
+
 # -- metadata enrichment (plan §6.2) -----------------------------------------
 class AudioTagReader(Protocol):
     """Reads embedded tags from a local audio file. Returns ``None`` only when