fix: run 7464's two failures — an unused loop target and a missed call site (4392)

Both mine, both from the same change.

`identity` is unpacked in the write loop and never read there — the decision
it feeds happens above it, when `conclusive` is built. flake8-bugbear is on
repo-wide and B007 is exactly this. Renamed `_identity`, and `linked +=
status == "linked"` spelled out as the `if` it actually is.

The second is worse, because it was a real assertion silently pointed at the
wrong shape. `match_post` now returns `(proposed, linked)`, and when I
rewrote the call sites I matched on `) == 0` — so the one comparison in the
file that reads `) == 1` kept comparing a tuple to an integer. Found by
walking the module's AST for every comparison against `match_post` and every
use of a name assigned from it, rather than grepping again with the pattern
that had already missed it once.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LVjrnpQjRgHdvq95rASoiR

backend/app/services/post_association_service.py

@@ -391,9 +391,10 @@ class PostAssociationService:
                 auto_id = conclusive[0][0].id
 
         linked = 0
-        for group, score, signals, identity in scored:
+        for group, score, signals, _identity in scored:
             status = "linked" if group.id == auto_id else "pending"
-            linked += status == "linked"
+            if status == "linked":
+                linked += 1
             self.session.add(PostAssociation(
                 announcement_post_id=announcement.id,
                 payload_post_id=group.id,

tests/test_post_association.py

@@ -306,7 +306,7 @@ async def test_a_dismissed_pair_is_never_proposed_again(db):
     svc = PostAssociationService(db)
     assert await svc.match_post(
         teaser.id, threshold=DEFAULT_THRESHOLD, window_hours=WINDOW,
-    ) == 1
+    ) == (1, 0)
     await db.commit()
 
     assoc = (await db.execute(select(PostAssociation))).scalar_one()