fix(wip-title): count inserts via pre-SELECT, not driver rowcount

apply_wip_image_tags relied on result.rowcount, but psycopg reports -1 for a
multi-row INSERT ... ON CONFLICT DO NOTHING (executemany path), so the return
count (and the backfill's reported total) was wrong. Compute the count from a
pre-SELECT of already-tagged ids within the same transaction; keep ON CONFLICT
DO NOTHING as a race-safety belt.

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

backend/app/services/wip_title.py

@@ -65,22 +65,37 @@ def resolve_wip_tag_id(session: Session) -> int | None:
 
 
 def apply_wip_image_tags(session: Session, image_ids, tag_id: int) -> int:
-    """Attach ``tag_id`` (source='wip_title') to each image id, idempotently
-    (ON CONFLICT DO NOTHING — never disturbs an existing tag or its source).
-    Returns the number of image_tag rows newly inserted. Does NOT commit."""
+    """Attach ``tag_id`` (source='wip_title') to each image id, idempotently —
+    never disturbs an existing tag or its source. Returns the number of image_tag
+    rows newly inserted. Does NOT commit.
+
+    The insert count is computed from a pre-SELECT of already-tagged ids rather
+    than the statement's ``rowcount``: psycopg reports -1 for a multi-row
+    ON CONFLICT DO NOTHING insert (it runs via an executemany path), so rowcount
+    is unusable here. The SELECT is accurate within this single transaction (no
+    concurrent writer touches these (image, wip) rows); ON CONFLICT DO NOTHING
+    stays as a race-safety belt so a rare concurrent insert can't error."""
     ids = list({int(i) for i in image_ids})
     if not ids:
         return 0
     inserted = 0
     for start in range(0, len(ids), _INSERT_CHUNK):
         chunk = ids[start:start + _INSERT_CHUNK]
-        result = session.execute(
+        already = set(session.execute(
+            select(image_tag.c.image_record_id)
+            .where(image_tag.c.tag_id == tag_id)
+            .where(image_tag.c.image_record_id.in_(chunk))
+        ).scalars())
+        to_insert = [iid for iid in chunk if iid not in already]
+        if not to_insert:
+            continue
+        session.execute(
             pg_insert(image_tag)
             .values([
                 {"image_record_id": iid, "tag_id": tag_id, "source": WIP_TITLE_SOURCE}
-                for iid in chunk
+                for iid in to_insert
             ])
             .on_conflict_do_nothing(index_elements=["image_record_id", "tag_id"])
         )
-        inserted += result.rowcount or 0
+        inserted += len(to_insert)
     return inserted