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_import.py

@@ -101,6 +101,25 @@ def test_sidecar_creates_provenance(importer, import_layout):
     assert rec.effective_date == post.post_date
 
 
+def test_sidecar_source_url_persists_filehash(importer, import_layout):
+    """#830 Phase 2: a media sidecar's source_url lands on the ImageRecord as
+    source_url + the lowercased CDN filehash (the inline-image join key)."""
+    import_root, _ = import_layout
+    m = import_root / "Alice" / "b.jpg"
+    _split(m, "v")
+    _sidecar(m, {
+        "category": "patreon", "id": 901, "title": "Body post",
+        "source_url": "https://cdn.test/p/0123456789ABCDEF0123456789abcdef/b.jpg",
+    })
+    r = importer.import_one(m)
+    assert r.status == "imported"
+    rec = importer.session.get(ImageRecord, r.image_id)
+    assert rec.source_url == (
+        "https://cdn.test/p/0123456789ABCDEF0123456789abcdef/b.jpg"
+    )
+    assert rec.source_filehash == "0123456789abcdef0123456789abcdef"
+
+
 def test_reimport_same_post_idempotent(importer, import_layout):
     import_root, _ = import_layout
     # Threshold 0: only an exact phash match collapses. Orthogonal splits