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/workers/tasks/download_task.py

@@ -0,0 +1,151 @@
+"""arq task: download one queued job through a fetch source (plan §6.1).
+
+Flow: load job → ``downloading`` → ``backend.fetch`` (progress streamed to the
+job row) → ``enriching`` → store file + minimal track → ``done`` → enqueue
+enrichment. yt-dlp fails often, so a failed fetch retries with exponential
+backoff (``download_max_retries``); only after the last try is the job marked
+``failed`` with a reason for the §A5 download manager.
+
+Heavy I/O belongs off the request cycle (CLAUDE.md); the HTTP endpoint only
+enqueues. The job row tolerates being deleted mid-flight (cancellation) — status
+writes against a missing row are no-ops.
+"""
+
+import uuid
+from typing import Any
+
+from arq import Retry
+
+from app.application.download_service import DownloadService
+from app.core.config import get_settings
+from app.core.logging import correlation_id, get_logger
+from app.domain.entities.download import DownloadJob
+from app.domain.errors import NotFoundError, ValidationError
+from app.domain.ports import FetchableSource, ProgressCallback
+from app.domain.sources import DownloadResult
+from app.infrastructure.db import session_scope
+from app.infrastructure.db.repositories import (
+    SqlAlchemyArtistRepository,
+    SqlAlchemyDownloadJobRepository,
+    SqlAlchemyTrackRepository,
+)
+from app.infrastructure.sources.registry import build_source_registry
+from app.infrastructure.storage.provider import get_file_storage
+from app.workers.queue import enqueue_enrich
+
+log = get_logger("worker.download")
+
+# Exponential backoff between retries: 30s, 60s, 120s … capped.
+_BACKOFF_BASE_SECONDS = 30
+_BACKOFF_MAX_SECONDS = 600
+# Only write progress when it advances by at least this much (avoid hammering
+# the DB on every yt-dlp chunk).
+_PROGRESS_STEP = 0.01
+
+
+async def download_track(_ctx: dict[str, Any], *, job_id: str) -> dict[str, Any]:
+    correlation_id.set(f"dl:{job_id}")
+    jid = uuid.UUID(job_id)
+    settings = get_settings()
+
+    job = await _load_job(jid)
+    if job is None:
+        log.info("download_job_missing", job_id=job_id)  # cancelled before pickup
+        return {"job_id": job_id, "status": "missing"}
+
+    registry = build_source_registry(settings)
+    try:
+        backend = registry.fetchable(job.source)
+    except (NotFoundError, ValidationError) as exc:
+        await _mark_failed(jid, f"Source unavailable: {exc}")
+        return {"job_id": job_id, "status": "failed"}
+    if job.source_id is None:
+        await _mark_failed(jid, "Job has no source_id to download.")
+        return {"job_id": job_id, "status": "failed"}
+
+    await _set_status(jid, "downloading")
+    try:
+        result = await _run_fetch(backend, job.source_id, jid)
+    except Exception as exc:
+        return await _handle_failure(jid, exc, settings.download_max_retries, job_id)
+
+    try:
+        track_id = await _import_result(jid, job, result)
+    except Exception as exc:
+        log.exception("download_import_failed", job_id=job_id)
+        await _mark_failed(jid, f"Import failed: {type(exc).__name__}: {exc}")
+        return {"job_id": job_id, "status": "failed"}
+
+    await enqueue_enrich(track_id)
+    log.info("download_complete", job_id=job_id, track_id=str(track_id))
+    return {"job_id": job_id, "status": "done", "track_id": str(track_id)}
+
+
+async def _run_fetch(
+    backend: FetchableSource, source_id: str, jid: uuid.UUID
+) -> DownloadResult:
+    """Fetch the file, streaming progress into the job row. A single session is
+    held for the download so progress writes don't churn connections; each
+    throttled update is committed so API pollers see it."""
+    async with session_scope() as session:
+        repo = SqlAlchemyDownloadJobRepository(session)
+        last = 0.0
+
+        async def on_progress(frac: float) -> None:
+            nonlocal last
+            if frac - last < _PROGRESS_STEP:
+                return
+            last = frac
+            await repo.set_progress(jid, frac)
+            await session.commit()
+
+        cb: ProgressCallback = on_progress
+        return await backend.fetch(source_id, on_progress=cb)
+
+
+async def _import_result(jid: uuid.UUID, job: DownloadJob, result: DownloadResult) -> uuid.UUID:
+    async with session_scope() as session:
+        repo = SqlAlchemyDownloadJobRepository(session)
+        await repo.set_status(jid, status="enriching")
+        service = DownloadService(
+            jobs=repo,
+            tracks=SqlAlchemyTrackRepository(session),
+            artists=SqlAlchemyArtistRepository(session),
+            storage=get_file_storage(),
+        )
+        track_id = await service.store_result(
+            source=job.source, result=result, requested_by=job.requested_by
+        )
+        await repo.set_status(jid, status="done", track_id=track_id)
+    return track_id
+
+
+async def _handle_failure(
+    jid: uuid.UUID, exc: Exception, max_retries: int, job_id: str
+) -> dict[str, Any]:
+    async with session_scope() as session:
+        tries = await SqlAlchemyDownloadJobRepository(session).increment_retry(jid)
+    if tries <= max_retries:
+        backoff = min(_BACKOFF_BASE_SECONDS * 2 ** (tries - 1), _BACKOFF_MAX_SECONDS)
+        log.warning("download_retry", job_id=job_id, attempt=tries, defer=backoff)
+        raise Retry(defer=backoff) from exc
+    log.exception("download_failed", job_id=job_id)
+    await _mark_failed(jid, f"Download failed after {tries} attempts: {type(exc).__name__}: {exc}")
+    return {"job_id": job_id, "status": "failed"}
+
+
+async def _load_job(jid: uuid.UUID) -> DownloadJob | None:
+    async with session_scope() as session:
+        return await SqlAlchemyDownloadJobRepository(session).get_by_id(jid)
+
+
+async def _set_status(jid: uuid.UUID, status: str) -> None:
+    async with session_scope() as session:
+        await SqlAlchemyDownloadJobRepository(session).set_status(jid, status=status)
+
+
+async def _mark_failed(jid: uuid.UUID, error: str) -> None:
+    async with session_scope() as session:
+        await SqlAlchemyDownloadJobRepository(session).set_status(
+            jid, status="failed", error_message=error
+        )