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>

tests/test_sidecar_import.py

@@ -120,6 +120,44 @@ def test_sidecar_source_url_persists_filehash(importer, import_layout):
     assert rec.source_filehash == "0123456789abcdef0123456789abcdef"
 
 
+def test_relink_source_filehash_backfills_existing(importer, import_layout):
+    """#830 recapture (Part B): relink_source_filehash backfills source_url +
+    source_filehash on an existing on-disk image's ImageRecord (matched by
+    sha256) WITHOUT re-importing or unlinking the file. NULL-only — never
+    clobbers a filehash already set."""
+    import_root, _ = import_layout
+    m = import_root / "Alice" / "img.jpg"
+    _split(m, "v")
+    r = importer.import_one(m)
+    assert r.status == "imported"
+    rec = importer.session.get(ImageRecord, r.image_id)
+    assert rec.source_filehash is None
+
+    url = "https://cdn.test/p/0123456789ABCDEF0123456789abcdef/img.jpg"
+    assert importer.relink_source_filehash(m, url) is True
+    importer.session.refresh(rec)
+    assert rec.source_url == url
+    assert rec.source_filehash == "0123456789abcdef0123456789abcdef"
+    assert m.exists()  # never unlinked
+
+    # NULL-only: a second relink with a different url is a no-op.
+    other = "https://cdn.test/p/ffffffffffffffffffffffffffffffff/x.jpg"
+    assert importer.relink_source_filehash(m, other) is False
+    importer.session.refresh(rec)
+    assert rec.source_filehash == "0123456789abcdef0123456789abcdef"
+
+
+def test_relink_source_filehash_no_match_is_noop(importer, import_layout):
+    """A path whose bytes match no ImageRecord (never imported) is a clean no-op,
+    not an error."""
+    import_root, _ = import_layout
+    m = import_root / "Alice" / "ghost.jpg"
+    _split(m, "h")
+    assert importer.relink_source_filehash(
+        m, "https://cdn.test/p/0123456789abcdef0123456789abcdef/ghost.jpg"
+    ) is False
+
+
 def test_reimport_same_post_idempotent(importer, import_layout):
     import_root, _ = import_layout
     # Threshold 0: only an exact phash match collapses. Orthogonal splits