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>

backend/app/services/importer.py

@@ -35,6 +35,7 @@ from ..utils import safe_probe
 from ..utils.paths import (
     derive_subdir,
     derive_top_level_artist,
+    filehash_from_url,
     hash_suffixed_name,
     safe_ext,
 )
@@ -1046,6 +1047,14 @@ class Importer:
         if record.artist_id is None:
             record.artist_id = artist.id
 
+        # #830 Phase 2: persist the file's CDN source identity (NULL-only, so a
+        # re-import / enrich-on-duplicate never clobbers it) — the filehash is
+        # the join key that lets post_feed_service remap the body's inline
+        # `<img src=CDN>` to this local copy at render time.
+        if sd.source_url and record.source_filehash is None:
+            record.source_url = sd.source_url
+            record.source_filehash = filehash_from_url(sd.source_url)
+
         if explicit_source is not None:
             src = explicit_source
         else: