feat(downloads): categorize download failures + a readable error dialog (S2)

Failed downloads stored the raw yt-dlp/ffmpeg text and dumped it as a truncated red
one-liner in the queue. Classify the failure into a category (unavailable / login_required /
geo_blocked / format_unavailable / drm / postprocess / source_missing / network / unknown)
and surface it properly:

- backend: app/downloads/errors.classify maps the raw message to a category code; the worker
  stores it in the new download_jobs.error_code (migration 0061) at both failure sites; resume
  clears it; the serializer exposes it. Raw text kept as technical detail.
- frontend: the queue row shows a clean localized reason + a "details" link opening a modal
  with the explanation, the raw text behind a disclosure, and a Retry only for transient
  categories (network/postprocess/unknown).
- tests: classify() unit coverage over verbatim yt-dlp/ffmpeg message shapes.

backend/app/worker.py

@@ -31,6 +31,7 @@ from app import sysconfig
 from app.config import settings
 from app.db import SessionLocal
 from app.downloads import edit as editmod
+from app.downloads import errors as dlerrors
 from app.downloads import formats, quota, service, storage
 from app.models import DownloadJob, MediaAsset
 from app.titles import normalize_title
@@ -654,6 +655,7 @@ def _download(job_id: int, asset_id: int, source_kind: str, source_ref: str, spe
             if job:
                 job.status = "error"
                 job.error = msg
+                job.error_code = dlerrors.classify(msg)
                 job.phase = None
             db.commit()
         log.warning("job %s failed: %s", job_id, msg)
@@ -685,6 +687,7 @@ def _fail_edit(job_id: int, asset_id: int, msg: str) -> None:
         if job:
             job.status = "error"
             job.error = msg
+            job.error_code = dlerrors.classify(msg)
             job.phase = None
         db.commit()
     log.warning("edit job %s failed: %s", job_id, msg)