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/importer.py

@@ -23,6 +23,7 @@ from sqlalchemy.orm import Session
 
 from ..models import (
     Artist,
+    ExternalLink,
     ImageProvenance,
     ImageRecord,
     ImportSettings,
@@ -39,6 +40,7 @@ from ..utils.paths import (
 )
 from ..utils.phash import compute_phash, find_similar
 from ..utils.sidecar import find_sidecar, parse_sidecar
+from .link_extract import extract_external_links
 from ..utils.slug import slugify
 from .archive_extractor import extract_archive, is_archive
 from .attachment_store import AttachmentStore
@@ -305,6 +307,32 @@ class Importer:
             ),
         )
 
+    def _sync_external_links(self, post: Post) -> None:
+        """Record off-platform file-host links (mega/gdrive/mediafire/dropbox/
+        pixeldrain) found in the post body, so they're never silently dropped
+        and the download worker can fetch them later. Shared by every platform's
+        import (runs off Post.description). INSERT-MISSING only — an existing row
+        keeps its status/attempts (a re-import must not reset a link already
+        downloaded or dead-lettered). Identity is (post_id, url); the full url
+        incl. #fragment is preserved by the extractor."""
+        links = extract_external_links(post.description)
+        if not links:
+            return
+        self.session.flush()  # ensure post.id is assigned before we reference it
+        existing = set(self.session.execute(
+            select(ExternalLink.url).where(ExternalLink.post_id == post.id)
+        ).scalars().all())
+        for link in links:
+            if link.url in existing:
+                continue
+            self.session.add(ExternalLink(
+                post_id=post.id,
+                artist_id=post.artist_id,
+                host=link.host,
+                url=link.url,
+                label=link.label,
+            ))
+
     def import_one(self, source: Path) -> ImportResult:
         """Dispatch by kind. Media → normal pipeline. Archive → extract
         media members (one Post via the archive-adjacent sidecar) and
@@ -767,6 +795,7 @@ class Importer:
         if sd.attachment_count is not None:
             post.attachment_count = sd.attachment_count
         post.raw_metadata = sd.raw
+        self._sync_external_links(post)
         self.session.commit()
         return True
 
@@ -1042,6 +1071,7 @@ class Importer:
         if sd.attachment_count is not None:
             post.attachment_count = sd.attachment_count
         post.raw_metadata = sd.raw
+        self._sync_external_links(post)
 
         # Race-safe (image_record_id, post_id) upsert — mirrors the
         # _find_or_create_source/post savepoint pattern. The plain