feat(ingest): localize inline post-body images to local copies (Phase 2)

Render a post body faithfully by serving our stored copies of inline
images instead of hotlinking the public CDN. The join key is the CDN
filehash (32-hex MD5) shared between a body <img src> and the media URL
we downloaded (the same identity extract_media dedups by):

- utils.paths.filehash_from_url — one source of truth for the extractor;
  patreon_client._filehash now delegates so capture- and render-time
  hashing cannot drift.
- ImageRecord gains source_url (provenance) + source_filehash (indexed
  match key); migration 0051.
- the per-media sidecar carries the file's source_url; the importer
  persists it (NULL-only) on the ImageRecord via _apply_sidecar.
- post_feed_service.get_post remaps body <img src> -> /images/<path> for
  every inline image whose filehash maps to a stored image of THIS
  artist; unmatched / pre-Phase-2 images keep hotlinking.

Pre-existing on-disk images have no filehash yet, so they fall back to
hotlinking until re-downloaded; localization is forward-looking.

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

tests/test_patreon_downloader.py

@@ -175,6 +175,9 @@ def test_sidecar_written_and_findable(tmp_path):
     assert data["id"] == "1001"
     assert data["title"] == "My Post Title"
     assert data["url"] == "https://www.patreon.com/posts/1001"
+    # #830 Phase 2: the per-media sidecar records THIS file's CDN URL so the
+    # importer can persist its filehash for inline-image localization.
+    assert data["source_url"] == "https://cdn.patreon.com/media1.png"
 
 
 def test_skip_seen(tmp_path):