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>

alembic/versions/0051_image_source_provenance.py

@@ -0,0 +1,38 @@
+"""image_record: source_url + source_filehash (inline-image localization)
+
+#830 Phase 2. To render a post body faithfully we serve LOCAL copies of inline
+images instead of hotlinking the public CDN. The join key between a body
+`<img src=CDN>` and the local file is the CDN's 32-hex filehash (the same
+identity extract_media dedups by). Persist it (indexed) plus the full source
+URL for provenance/debugging. Both NULL for filesystem-imported / pre-existing
+rows — those fall back to hotlinking until re-downloaded.
+
+Revision ID: 0051
+Revises: 0050
+Create Date: 2026-06-14
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0051"
+down_revision: Union[str, None] = "0050"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column("image_record", sa.Column("source_url", sa.Text(), nullable=True))
+    op.add_column(
+        "image_record", sa.Column("source_filehash", sa.String(length=32), nullable=True)
+    )
+    op.create_index(
+        "ix_image_record_source_filehash", "image_record", ["source_filehash"]
+    )
+
+
+def downgrade() -> None:
+    op.drop_index("ix_image_record_source_filehash", table_name="image_record")
+    op.drop_column("image_record", "source_filehash")
+    op.drop_column("image_record", "source_url")