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/utils/paths.py

@@ -1,9 +1,28 @@
 """Filesystem path helpers — destination derivation, hash-suffixed names."""
 
+import re
 from pathlib import Path
 
 _MAX_EXT_LEN = 16
 
+# A Patreon/gallery CDN URL embeds a 32-char hex (MD5) path segment that is the
+# file's stable per-file identity — the same role gallery-dl's `_filehash`
+# plays. It is the join key between a post body `<img src=CDN>` and the local
+# copy we downloaded (extract_media dedups content vs gallery images by it), so
+# this ONE extractor must be used for both capture-time persistence and
+# render-time matching — they cannot be allowed to drift. Match the FIRST 32-hex
+# run anywhere in the URL (path or query); real CDN URLs carry exactly one.
+_FILEHASH_RE = re.compile(r"([0-9a-fA-F]{32})")
+
+
+def filehash_from_url(url: str | None) -> str | None:
+    """The 32-char hex (MD5) CDN identity segment of `url`, lowercased, or None
+    when the URL is empty / carries no such segment."""
+    if not url:
+        return None
+    match = _FILEHASH_RE.search(url)
+    return match.group(1).lower() if match else None
+
 
 def safe_ext(name: str | Path) -> str:
     """Conservatively extract a short, alphanumeric file extension.