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_sidecar_util.py

@@ -119,6 +119,15 @@ def test_parse_message_used_as_description_fallback():
     assert sd.description == "hello channel"
 
 
+def test_parse_source_url_present_and_absent():
+    """The per-media sidecar carries a `source_url` (#830 Phase 2); a post-only
+    sidecar (no media file) omits it → None."""
+    sd = parse_sidecar({"category": "patreon", "id": "1",
+                        "source_url": "https://cdn.test/abc.png"})
+    assert sd.source_url == "https://cdn.test/abc.png"
+    assert parse_sidecar({"category": "patreon", "id": "1"}).source_url is None
+
+
 def test_parse_date_epoch_and_unparseable_and_naive():
     assert parse_sidecar({"timestamp": 1690857602}).post_date.tzinfo is not None
     assert parse_sidecar({"date": "not-a-date"}).post_date is None