fix(jobs): match the boolean partial predicates to the IS TRUE/IS FALSE queries (E2 S6 review)

The via_search / shorts_probed partials used `WHERE via_search` / `WHERE NOT
shorts_probed`, but the queries filter with `.is_(True)`/`.is_(False)` (SQL
`IS TRUE`/`IS FALSE`), which Postgres's partial-index prover does NOT match to a
bare-boolean predicate — so both fell back to a full seq scan (the shorts-probe
job kept full-scanning for its work queue, the exact thing PERF-3 meant to fix).

Align the predicates to the query form: `WHERE via_search IS TRUE` and
`WHERE shorts_probed IS FALSE`. Verified on the prod-sized localdev: both now
Index Only Scan (via_search 44,683 -> 3 buffers; shorts_probed 44,683 -> 1).
Gate green (346 pytest).

backend/alembic/versions/0069_job_index_hygiene.py

@@ -45,14 +45,17 @@ def upgrade() -> None:
         op.execute("DROP INDEX CONCURRENTLY IF EXISTS ix_videos_is_short")
         op.execute("DROP INDEX CONCURRENTLY IF EXISTS ix_videos_via_explore")
         op.execute("DROP INDEX CONCURRENTLY IF EXISTS ix_videos_via_search")
+        # Predicate MUST match the query's `.is_(True)`/`.is_(False)` form (SQL `IS TRUE`/`IS FALSE`):
+        # Postgres's partial-index prover does NOT match a `col IS TRUE` clause to a bare `col` predicate,
+        # so `WHERE via_search` / `WHERE NOT shorts_probed` would leave the queries seq-scanning.
         op.execute(
             "CREATE INDEX CONCURRENTLY IF NOT EXISTS ix_videos_via_search "
-            "ON videos (id) WHERE via_search"
+            "ON videos (id) WHERE via_search IS TRUE"
         )
         op.execute("DROP INDEX CONCURRENTLY IF EXISTS ix_videos_shorts_probed")
         op.execute(
             "CREATE INDEX CONCURRENTLY IF NOT EXISTS ix_videos_shorts_probed "
-            "ON videos (id) WHERE NOT shorts_probed"
+            "ON videos (id) WHERE shorts_probed IS FALSE"
         )
         op.execute("DROP INDEX CONCURRENTLY IF EXISTS ix_videos_unavailable_since")
         op.execute(