fix(posts): link duplicate items to every post + prune bare shells

The native Patreon backfill flooded the feed with bare 'Post <id>' shells
(1589 for Anduo). Root cause: PostAttachment.sha256 was GLOBALLY unique, so a
non-art file reused across posts only ever linked to the first one, and
_capture_attachment created the Post before that dedup check — leaving later
posts with no image and no attachment. Duplicate IMAGES had the mirror gap:
attach_in_place returned duplicate_hash/duplicate_phash before _apply_sidecar,
so the second post got no provenance row, and the feed only rendered via
primary_post_id (one post per image).

Operator requirement: a duplicate item must show on EVERY post it appears in.
Unify the fix as link-not-suppress:

- importer: on duplicate_hash / duplicate_phash(larger_exists), append an
  image_provenance row for the new post (keep primary on the first). Both the
  download path (attach_in_place) and the filesystem path (_import_media).
- post_feed_service: render thumbnails by image_provenance UNION primary_post_id,
  so a cross-posted image shows on every post (and legacy primary-only images
  still show).
- PostAttachment: per-post uniqueness — drop UNIQUE(sha256), add partial
  UNIQUE(post_id, sha256) + partial UNIQUE(sha256) WHERE post_id IS NULL
  (migration 0043); _capture_attachment dedups per-(post,sha) over the shared
  sha-addressed blob, so no post is left bare.
- cleanup: new prune-bare-posts maintenance action (cleanup_service
  _bare_post_conditions shared by preview/count/delete per preview/apply parity;
  admin endpoint; PostMaintenanceCard). Deletes posts with zero image links
  (primary or provenance) AND zero attachments. Run after the feed fix so a
  hidden provenance link spares the post instead of deleting it.

Tests: dup image shows on both posts; dup attachment shows on both posts; feed
renders provenance-linked duplicates; prune-bare delete-path == preview.

Operator redeploys (migration 0043) then runs the prune to clear the shells.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

alembic/versions/0043_post_attachment_per_post_unique.py

@@ -0,0 +1,62 @@
+"""post_attachment: per-post sha uniqueness (empty-post flood fix)
+
+Revision ID: 0043
+Revises: 0042
+Create Date: 2026-06-08
+
+PostAttachment.sha256 was GLOBALLY unique, so a non-art file the creator attaches
+to many posts (a standard pdf/zip/link-card) only ever got ONE row — on the first
+post — leaving every later post a bare shell (no image, no attachment). The native
+Patreon backfill of Anduo surfaced 1589 such shells (operator-flagged 2026-06-08).
+
+Switch to PER-POST uniqueness: the on-disk blob stays sha-deduped, but each post
+gets its own row. Replace the unique sha256 index with a plain lookup index plus
+two partial uniques — (post_id, sha256) for real posts and (sha256) for the
+NULL-post filesystem case (still one row per file there).
+
+Existing data has ≤1 row per sha (the old global unique), so the new partial
+uniques can't be violated on upgrade — no data backfill needed here. The bare-post
+shells themselves are removed by the separate prune-empty-posts cleanup tool.
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0043"
+down_revision: Union[str, None] = "0042"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    # Drop the global unique index; recreate it as a plain (non-unique) lookup
+    # index so sha-based reads keep their index (matches the model's index=True).
+    op.drop_index("ix_post_attachment_sha256", table_name="post_attachment")
+    op.create_index(
+        "ix_post_attachment_sha256", "post_attachment", ["sha256"],
+    )
+    op.create_index(
+        "uq_post_attachment_post_sha", "post_attachment",
+        ["post_id", "sha256"], unique=True,
+        postgresql_where=sa.text("post_id IS NOT NULL"),
+    )
+    op.create_index(
+        "uq_post_attachment_null_post_sha", "post_attachment",
+        ["sha256"], unique=True,
+        postgresql_where=sa.text("post_id IS NULL"),
+    )
+
+
+def downgrade() -> None:
+    op.drop_index(
+        "uq_post_attachment_null_post_sha", table_name="post_attachment"
+    )
+    op.drop_index(
+        "uq_post_attachment_post_sha", table_name="post_attachment"
+    )
+    op.drop_index("ix_post_attachment_sha256", table_name="post_attachment")
+    op.create_index(
+        "ix_post_attachment_sha256", "post_attachment", ["sha256"],
+        unique=True,
+    )