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/html_sanitize.py

@@ -6,6 +6,8 @@ server-side to a tight allowlist. nh3 (Rust/ammonia bindings) is used;
 bleach is deprecated upstream and intentionally not used.
 """
 
+import re
+
 import nh3
 
 # A faithful-but-safe set: enough to reproduce the SEMANTIC look of a scraped
@@ -50,3 +52,39 @@ def sanitize_post_html(raw: str | None) -> str | None:
         link_rel="noopener noreferrer",
         clean_content_tags={"script", "style"},
     )
+
+
+# nh3 emits attributes double-quoted, so a single `src="..."` capture is enough
+# to read/rewrite inline-image sources on already-sanitized markup (#830 Phase 2).
+_IMG_SRC_RE = re.compile(r'(<img\b[^>]*?\bsrc=")([^"]*)(")', re.IGNORECASE)
+
+
+def extract_img_srcs(html: str | None) -> list[str]:
+    """The raw `src` values of every <img> in `html`, in document order, deduped
+    (first kept). Returned EXACTLY as they appear (still entity-escaped) so a
+    caller can match them back for verbatim substitution."""
+    if not html:
+        return []
+    out: list[str] = []
+    seen: set[str] = set()
+    for _pre, src, _post in _IMG_SRC_RE.findall(html):
+        if src and src not in seen:
+            seen.add(src)
+            out.append(src)
+    return out
+
+
+def rewrite_img_srcs(html: str | None, replace: dict[str, str]) -> str | None:
+    """Swap each <img src> whose raw (as-in-HTML) value is a key in `replace`
+    with the mapped value; others are left untouched. Used to point a post body's
+    inline images at locally-served copies (#830 Phase 2). Operates only on src
+    values of already-sanitized markup, so the result stays within the allowlist.
+    Returns `html` unchanged when it or `replace` is empty."""
+    if not html or not replace:
+        return html
+
+    def _sub(m: re.Match) -> str:
+        pre, src, post = m.group(1), m.group(2), m.group(3)
+        return f"{pre}{replace.get(src, src)}{post}"
+
+    return _IMG_SRC_RE.sub(_sub, html)