feat: a teaser's card references the drop it announced and the piece's variants (4402, 4401)

The operator's problem: a Patreon teaser is a pointer, and its card showed the
censored crop plus a text link while the content it pointed at sat on another
card. The fix is a REFERENCE, not an absorption: "the nested items on the
unified post are a duplicate or reference of existing content". Nothing is
written. Discord posts keep their own rows, dates and places in the feed.

- post_unification: for each teaser with a linked association, the drop's
  images and text, plus its variant family: Discord images sharing the
  seed's gated LEADING working name, or a phash near-duplicate, within a
  window of the teaser. One hop only, oldest first.
- Measured on artist 8 before writing it: of 121 message pairs 2-60 days
  apart that share a gated token, 106 share the leading name and all read
  as real families. Of the 15 sharing only a trailing word, 14 are sibling
  pieces and one is a plain collision (`bottom`, 56 days). The family cap
  is 8, not the pairing cap of 6, because `tentacooler` and `0-k1` (6 posts
  each) are real families.
- The feed drops a linked drop's own card only within discord_link_fold_hours
  of its teaser (default 24): "only hidden from the post view they're posted
  the same day". Older referenced posts stay where they landed.
- post_association.linked_by records whether FC or a person made the link,
  so the card can say so. Undo is the existing dismiss.
- `image0` (gallery-dl's fallback name) becomes a stopword.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LVjrnpQjRgHdvq95rASoiR

backend/app/services/post_association_service.py

@@ -500,6 +500,7 @@ class PostAssociationService:
                 score=score,
                 signals=signals,
                 status=status,
+                linked_by="fc" if status == "linked" else None,
             ))
             made += 1
         return made, linked
@@ -526,14 +527,19 @@ class PostAssociationService:
         if a is None:
             return None
         a.status = "linked"
+        a.linked_by = "operator"
         return {"id": a.id, "status": a.status}
 
     async def dismiss(self, association_id: int) -> dict | None:
         a = await self.session.get(PostAssociation, association_id)
         if a is None:
             return None
-        # Kept, not deleted — the row is what remembers the rejection.
+        # Kept, not deleted — the row is what remembers the rejection. It is
+        # also the undo for a link FC made itself (#4402): the unified card
+        # dismisses the pair, and the dismissed row stops the next sweep from
+        # linking it straight back.
         a.status = "dismissed"
+        a.linked_by = None
         return {"id": a.id, "status": a.status}
 
     async def linked_for(self, post_ids: list[int]) -> dict[int, list[dict]]: