perf(tables): tenant-scoped containment index (migration 0232)

The plain GIN on user_table_rows.data matched @> candidates across every
tenant sharing the relation — a hot value in someone else's table
inflated everyone's equality filters (1.07M candidates fetched for a
33k-row match, lossy bitmap, 1.1s). Replace it with btree_gin
(table_id, data jsonb_path_ops): the tenant intersection happens inside
the index and paths are single hashed entries. Rare-equality probe
326ms -> 17ms with zero wasted candidates; unique-constraint checks and
upsert conflict lookups ride the same index. The new index is smaller
than the one it replaces (529MB vs 694MB on the 12M-row dev relation).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: TheodoreSpeaks

packages/db/migrations/0232_tenant_scoped_data_gin.sql

@@ -0,0 +1,8 @@
+-- Tenant-scoped containment index: a plain GIN on data matches @> candidates across every
+-- tenant sharing this relation (measured 1.07M candidates fetched for a 33k-row match). btree_gin
+-- lets table_id lead the GIN so the intersection happens inside the index, and jsonb_path_ops
+-- indexes whole key->value paths (single lookup, smaller index). Every @> query carries table_id,
+-- so the old cross-tenant index is strictly redundant.
+CREATE EXTENSION IF NOT EXISTS btree_gin;--> statement-breakpoint
+DROP INDEX "user_table_rows_data_gin_idx";--> statement-breakpoint
+CREATE INDEX "user_table_rows_tenant_data_gin_idx" ON "user_table_rows" USING gin ("table_id","data" jsonb_path_ops);