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/models/image_record.py

@@ -49,6 +49,18 @@ class ImageRecord(Base):
     # Thumbnail (populated by FC-2)
     thumbnail_path: Mapped[str | None] = mapped_column(Text, nullable=True)
 
+    # Source provenance for downloaded media (#830 Phase 2). `source_url` is the
+    # CDN/origin URL the file was fetched from (debugging + future re-fetch).
+    # `source_filehash` is the URL's 32-hex CDN identity segment
+    # (utils.paths.filehash_from_url) — the JOIN KEY that maps a post body's
+    # inline `<img src=CDN>` back to this local copy so the rendered body serves
+    # our stored image instead of hotlinking the public source. Indexed for the
+    # render-time lookup. NULL for filesystem-imported / pre-Phase-2 rows.
+    source_url: Mapped[str | None] = mapped_column(Text, nullable=True)
+    source_filehash: Mapped[str | None] = mapped_column(
+        String(32), nullable=True, index=True
+    )
+
     # Origin / provenance pointers
     origin: Mapped[str] = mapped_column(Enum(*ORIGIN_CHOICES, name="origin_enum"), nullable=False)
     primary_post_id: Mapped[int | None] = mapped_column(