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>

backend/app/models/post_attachment.py

@@ -14,10 +14,12 @@ from sqlalchemy import (
     BigInteger,
     DateTime,
     ForeignKey,
+    Index,
     Integer,
     String,
     Text,
     func,
+    text,
 )
 from sqlalchemy.orm import Mapped, mapped_column
 
@@ -26,6 +28,24 @@ from .base import Base
 
 class PostAttachment(Base):
     __tablename__ = "post_attachment"
+    # Dedup is PER-POST, not global (2026-06-08): the same non-art file attached
+    # to many posts gets one row per post over a single sha-addressed blob, so no
+    # post is left a bare shell. Partial uniques: (post_id, sha256) for real posts;
+    # (sha256) alone for the NULL-post filesystem case (one row per file there).
+    __table_args__ = (
+        Index(
+            "uq_post_attachment_post_sha",
+            "post_id", "sha256",
+            unique=True,
+            postgresql_where=text("post_id IS NOT NULL"),
+        ),
+        Index(
+            "uq_post_attachment_null_post_sha",
+            "sha256",
+            unique=True,
+            postgresql_where=text("post_id IS NULL"),
+        ),
+    )
 
     id: Mapped[int] = mapped_column(Integer, primary_key=True)
     post_id: Mapped[int | None] = mapped_column(
@@ -35,7 +55,7 @@ class PostAttachment(Base):
         ForeignKey("artist.id", ondelete="SET NULL"), nullable=True, index=True
     )
     sha256: Mapped[str] = mapped_column(
-        String(64), nullable=False, unique=True, index=True
+        String(64), nullable=False, index=True
     )
     path: Mapped[str] = mapped_column(Text, nullable=False)
     original_filename: Mapped[str] = mapped_column(Text, nullable=False)