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/patreon_client.py

@@ -39,7 +39,7 @@ from urllib.parse import parse_qs, urlsplit
 
 import requests
 
-from ..utils.paths import safe_ext
+from ..utils.paths import filehash_from_url, safe_ext
 
 log = logging.getLogger(__name__)
 
@@ -90,12 +90,6 @@ _FIELDS_POST = (
 _FIELDS_MEDIA = "id,image_urls,download_url,metadata,file_name"
 _FIELDS_CAMPAIGN = "name,url"
 
-# A CDN download URL embeds a 32-char hex (MD5) path segment; that segment is
-# Patreon's stable per-file identity and is what we dedup + ledger against.
-# Same role gallery-dl's _filehash plays. Match the FIRST 32-hex run anywhere
-# in the URL (path or query); real Patreon CDN URLs carry exactly one.
-_FILEHASH_RE = re.compile(r"([0-9a-fA-F]{32})")
-
 # Inline post `content` is HTML; images are emitted as <img ... src="...">.
 # Pull every src; downstream dedup collapses any that duplicate a gallery item
 # by filehash. Tolerant of attribute ordering and single/double quotes.
@@ -188,10 +182,9 @@ def _load_session(cookies_path: str | Path | None) -> requests.Session:
 
 
 def _filehash(url: str) -> str | None:
-    if not url:
-        return None
-    match = _FILEHASH_RE.search(url)
-    return match.group(1).lower() if match else None
+    # Delegate to the shared extractor (utils.paths) so capture-time persistence
+    # and render-time inline-image matching use the EXACT same identity.
+    return filehash_from_url(url)
 
 
 def _basename_from_url(url: str) -> str: