fix(downloads): force IPv4 (fixes container 'Errno -5' DNS failures) + retry resets errored asset

Root cause of the failing downloads: containers usually have no working IPv6 route, but
googlevideo CDN hosts advertise AAAA records, so the downloader tried IPv6 and failed with
'[Errno -5] No address associated with hostname'. Small clips happened to use IPv4; long videos
(more CDN requests / the ffmpeg path) hit IPv6 and died.
- yt-dlp source_address=0.0.0.0 forces IPv4 for every connection (== --force-ipv4). Verified: the
  full video+audio download completes cleanly.
- Also: retry (resume) now resets the shared asset from 'error' to 'pending', so it actually
  re-downloads — previously the requeued job instantly re-inherited the asset's stale error
  because the worker short-circuits a job whose asset already errored.
- Dropped the localdev DNS override (wrong hypothesis; the issue was IPv6, not the resolver).
- Kept yt-dlp retries/fragment_retries/socket_timeout for transient blips.

Verified end-to-end in the worker: two previously-failing long videos now download to done.

backend/app/downloads/formats.py

@@ -91,10 +91,17 @@ def build_ydl_opts(spec: dict, outtmpl: str, progress_hook) -> dict:
         "writethumbnail": True,  # kept for the poster.jpg sidecar (and embed, if requested)
         "progress_hooks": [progress_hook],
         "postprocessors": [],
+        # Force IPv4: containers often have no working IPv6 route, but googlevideo CDN hosts
+        # advertise AAAA records, so ffmpeg/the downloader would try IPv6 and fail with
+        # "[Errno -5] No address associated with hostname". Binding to the IPv4 any-address
+        # makes every connection IPv4-only. (yt-dlp's --force-ipv4 does the same thing.)
+        "source_address": "0.0.0.0",
         # Self-heal transient network blips instead of failing the whole job.
-        "retries": 3,
-        "fragment_retries": 5,
-        "retry_sleep": {"http": 2, "fragment": 2},
+        "retries": 10,
+        "fragment_retries": 10,
+        "extractor_retries": 3,
+        "retry_sleep": {"http": 5, "fragment": 5, "extractor": 5},
+        "socket_timeout": 30,
     }
 
     if s["mode"] == "a":