feat: link the Patreon teaser to the Discord drop it announced (388 step E5)

The point of the milestone rather than its tail. Two of the operator's artists
post a deliberately cropped fragment on Patreon to signal that the real thing
has landed in their Discord; this proposes those pairs.

Confirm-only, following the FC-6.3 series matcher. A wrongly-asserted
association tells the operator two different pieces are one, which is strictly
worse than no link: no link leaves them where they already were, a wrong one
actively misinforms and then propagates into whatever reads it. So the
matcher's job is a SHORT list worth reading, not a long list worth trusting.

**The threshold sits above every single signal weight, and that is the
design.** Proximity is 0.55, declaration 0.45, the cut 0.60 — so neither
signal can carry a pair alone. That makes "time proximity alone is never
sufficient" an arithmetic property rather than an aspiration: on a busy day an
artist posts several times, and a matcher that could pair on proximity alone
would turn every one of those days into false pairs until the review queue got
abandoned. A guard test asserts the relationship against WEIGHTS directly, so
it survives any refactor of the scorer, and says in its own failure message not
to fix it by lowering the assertion.

**Crop-to-source matching is HELD, on the plan's instruction** — real work with
real false-positive risk, worth building only once signals 1 and 2 are shown
insufficient against the operator's actual artists. Worth stating: a naive
whole-image SigLIP similarity is NOT that signal. A cropped teaser and its full
version are precisely the pair a whole-image comparison handles worst, so
adding one as a "bonus" would mostly add noise while looking like progress.

Two premises in the plan corrected in the building:

* **E4 is not actually a prerequisite.** A Patreon Source and a Discord Source
  the operator has added under one Artist already share `Post.artist_id`, and
  the synthetic grouping inherits it. E4 EXTENDS this to creators FC has to
  learn the association for; it is not needed to represent one FC was told.
  Same-artist is then a hard filter, not a scored signal — two different
  creators posting minutes apart is a coincidence, not evidence.
* **`link_extract` cannot supply the declaration signal.** It exists, but
  `SUPPORTED_HOSTS` is file hosts only and `host_for()` returns None for a
  Discord URL, so no ExternalLink row is ever written for one. The signal
  reads the post body directly instead.

And a bug my own test would have caught: `declared_signal` stripped the HTML
before looking for an invite, but `html_to_plain` discards attributes and
these creators put the invite in an anchor's `href` — so the strongest form of
the signal was being thrown away, leaving only whatever the link text said.
The invite now matches the raw body; the bare mention still matches stripped
text, so `\bdiscord\b` is tested against prose rather than against markup.

Dismissed rows are kept, not deleted: the row is what remembers the rejection,
and re-proposing a rejected pair on every scan is the one behaviour that makes
a review queue get ignored. Both FKs CASCADE, so E3's one-DELETE reversal
cannot leave a proposal pointing at a post that no longer exists.

Only ACCEPTED links reach the post payload. A pending proposal is a question
for the review queue, not a claim to render beside the artwork.

UI (rule 27) follows in the next commit.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LNXXULQDjVZmbuNa2G9mD9

backend/app/services/post_feed_service.py

@@ -175,9 +175,10 @@ class PostFeedService:
         post_ids = [p.id for p, _, _ in rows]
         thumbs_map = await self._thumbnails_for(post_ids)
         atts_map = await self._attachments_for(post_ids)
+        links_map = await self._links_for(post_ids)
 
         items = [
-            self._to_dict(post, artist, source, thumbs_map, atts_map)
+            self._to_dict(post, artist, source, thumbs_map, atts_map, links_map)
             for post, artist, source in rows
         ]
         return {"items": items, "next_cursor": next_cursor}
@@ -218,6 +219,7 @@ class PostFeedService:
         atts_map = await self._attachments_for([anchor_post.id])
         anchor_item = self._to_dict(
             anchor_post, anchor_artist, anchor_source, thumbs_map, atts_map,
+            await self._links_for([anchor_post.id]),
         )
         return {
             "items": newer["items"] + [anchor_item] + older["items"],
@@ -241,7 +243,10 @@ class PostFeedService:
         # the default arg.
         thumbs_map = await self._thumbnails_for([post.id], limit=None)
         atts_map = await self._attachments_for([post.id])
-        item = self._to_dict(post, artist, source, thumbs_map, atts_map)
+        item = self._to_dict(
+            post, artist, source, thumbs_map, atts_map,
+            await self._links_for([post.id]),
+        )
         item["description_full"] = html_to_plain(post.description)
         # Full (uncapped) translated description for the detail view (#143).
         item["description_translated_full"] = post.description_translated
@@ -407,9 +412,25 @@ class PostFeedService:
             })
         return out
 
+    async def _links_for(self, post_ids: list[int]) -> dict[int, list[dict]]:
+        """Accepted announcement links touching these posts (#388 E5).
+
+        Only ACCEPTED ones. A pending proposal is a question for the review
+        queue, not a claim to render beside the artwork — showing one here
+        would assert a link the operator has not agreed to, which is the exact
+        failure the confirm-only design exists to prevent.
+        """
+        # Imported here rather than at module scope: post_association_service
+        # imports discord_grouping, which imports the models, and the feed
+        # service is imported by the API at startup. A local import keeps that
+        # chain out of the import graph for a purely optional read.
+        from .post_association_service import PostAssociationService
+
+        return await PostAssociationService(self.session).linked_for(post_ids)
+
     def _to_dict(
         self, post: Post, artist: Artist, source: Source | None,
-        thumbs_map: dict, atts_map: dict,
+        thumbs_map: dict, atts_map: dict, links_map: dict | None = None,
     ) -> dict:
         plain_full = html_to_plain(post.description) if post.description else None
         if plain_full is None:
@@ -453,6 +474,11 @@ class PostFeedService:
             # ago" — which is the whole signal that chat content is trickling
             # in. NULL means it has not grown since it was created.
             "last_grew_at": post.last_grew_at.isoformat() if post.last_grew_at else None,
+            # Accepted links only (#388 E5): [{role, post_id, id}], where role
+            # is "announces" (this post is the teaser) or "announced_by" (this
+            # post is the drop). Always a list so the UI never branches on
+            # absence.
+            "associations": (links_map or {}).get(post.id, []),
             # Non-null on a chat message a synthetic post absorbed. The feed
             # filters these out, but `around`/`get_post` still reach them, and
             # the UI uses this to explain why a post it linked to is not in the