feat(ingest): Recapture mode — re-grab post bodies/links + localize on-disk inline images (#830)

A plain backfill gates post-body capture on the seen-ledger, so a post whose
media is already on disk AND whose post key is already seen never gets its body
recaptured (operator-flagged: Industrial Lust description missing). Recovery
recaptures unconditionally but re-downloads the whole source.

New 'recapture' walk mode (4th beside tick/backfill/recovery): bypasses the
post-record gate so EVERY post's body + external links are re-captured
(detail-fetching empty bodies) WITHOUT re-downloading on-disk media; and
surfaces already-present media via a separate non-deleting relink channel so the
importer backfills ImageRecord.source_filehash for inline-image localization.

- ingest_core: recapture mode + recapture_records gate bypass + relink collect
- patreon_downloader: recapture surfaces seen-on-disk as skipped_disk(path),
  never refetches seen-missing media, still downloads genuinely-new
- importer.relink_source_filehash: NULL-only sha256 backfill, never unlinks
- download_service: mode derivation + phase-3 relink loop + lifecycle clear
- source_service/api: start_recapture + backfill_recapture field + action
- frontend: Recapture kebab action + 'Recapturing' badge across SourceActions/
  Row/Card/SubscriptionsTab + sources store
- tests across ingester/downloader/importer/source_service/api/download_service

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/app/services/importer.py

@@ -1005,6 +1005,35 @@ class Importer:
                         return sv.strip()
         return None
 
+    def relink_source_filehash(
+        self, path: Path, source_url: str, *, artist: Artist | None = None,
+    ) -> bool:
+        """#830 recapture: backfill source_url + source_filehash on an EXISTING
+        on-disk image's ImageRecord (matched by sha256) so its post-body inline
+        `<img src=CDN>` remaps to the local copy at render time — WITHOUT
+        re-downloading and WITHOUT unlinking the file (unlike the import path's
+        duplicate-hash branch). NULL-only, mirroring _apply_sidecar: never
+        clobbers an already-set filehash. Returns True only when a row was
+        updated. No-op when the file is gone, the url has no filehash, no record
+        matches the bytes, or the record already carries a filehash.
+        """
+        fh = filehash_from_url(source_url)
+        if not fh:
+            return False
+        try:
+            sha = _sha256_of(path)
+        except OSError:
+            return False
+        record = self.session.execute(
+            select(ImageRecord).where(ImageRecord.sha256 == sha)
+        ).scalar_one_or_none()
+        if record is None or record.source_filehash is not None:
+            return False
+        record.source_url = source_url
+        record.source_filehash = fh
+        self.session.commit()
+        return True
+
     def _apply_sidecar(
         self,
         record: ImageRecord,