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/gallery_dl.py

@@ -153,6 +153,11 @@ class DownloadResult:
     # Importer.upsert_post_record (keyed on external_post_id → updates, never
     # doubles, the same Post a media import would create).
     post_record_paths: list[str] = field(default_factory=list)
+    # Native ingester, recapture mode only (#830): (on-disk path, CDN source_url)
+    # pairs for already-present media whose ImageRecord.source_filehash should be
+    # backfilled (inline-image localization) WITHOUT re-download or unlink. Empty
+    # on the gallery-dl path and outside recapture.
+    relink_source_paths: list[tuple[str, str]] = field(default_factory=list)
     stdout: str = ""
     stderr: str = ""
     return_code: int = 0