feat(posts): extract + record external file-host links (Phase 3)

Capture off-platform links (mega/gdrive/mediafire/dropbox/pixeldrain) embedded
in post bodies so they're never silently dropped, and surface them in the post
view. The download worker (Phase 4) walks these rows.

- link_extract.py: pure extractor — <a href> + bare URLs, unwraps Patreon
  redirect shims, PRESERVES the full url incl. #fragment (mega's key), dedups.
  Reusable by every platform (runs off Post.description).
- external_link model + migration 0049: post_id/artist_id/host/url/label/status
  /attempts/last_error/attachment_id/timing; CHECK whitelists (full enum incl.
  worker statuses up front) + (post_id,url) unique.
- importer._sync_external_links: insert-missing on both import paths
  (_apply_sidecar + upsert_post_record) so a re-import never resets a link's
  status; runs for all platforms.
- post_feed_service.get_post: returns external_links (detail-only).
- PostCard: renders the links (host chip + label + status) once expanded.
- tests: extractor (5 hosts, fragment, shim unwrap, dedup), importer (record +
  no-dup on reimport), serializer.

Refs FC #830.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/app/services/post_feed_service.py

@@ -16,6 +16,7 @@ from sqlalchemy.ext.asyncio import AsyncSession
 
 from ..models import (
     Artist,
+    ExternalLink,
     ImageProvenance,
     ImageRecord,
     Post,
@@ -196,8 +197,26 @@ class PostFeedService:
         # detail-only (the feed list stays lightweight plain text). None when the
         # post has no body.
         item["description_html"] = sanitize_post_html(post.description)
+        item["external_links"] = await self._external_links_for(post.id)
         return item
 
+    async def _external_links_for(self, post_id: int) -> list[dict]:
+        """Off-platform file-host links recorded for a post (detail-only). Each
+        carries its host, full url, label, and download status so the post view
+        can surface them (and, later, a retry/download affordance)."""
+        rows = (await self.session.execute(
+            select(ExternalLink)
+            .where(ExternalLink.post_id == post_id)
+            .order_by(ExternalLink.id.asc())
+        )).scalars().all()
+        return [
+            {
+                "id": e.id, "host": e.host, "url": e.url,
+                "label": e.label, "status": e.status,
+            }
+            for e in rows
+        ]
+
     # --- composition helpers ---------------------------------------------
 
     async def _thumbnails_for(