feat(db): add FTS GIN + savedSearch partial indexes (GOO-118)

Convert the query-optimization recommendations from GOO-57 audit plan
document into concrete Prisma migration changes. Neither index can be
expressed in Prisma schema (expression index / partial WHERE), so both
land as raw SQL in a single migration.

Indexes added:

- idx_property_fts — GIN expression index on Property matching the
  search-query-builder FTS_COLUMNS expression exactly:
    to_tsvector('simple', coalesce(title,'') || ' ' || coalesce(description,'')
                 || ' ' || coalesce(address,'') || ' ' || coalesce(district,'')
                 || ' ' || coalesce(city,''))
  Addresses GOO-57 M-3 (missing GIN index for FTS).

- idx_savedsearch_alert_enabled — partial btree on SavedSearch(createdAt)
  WHERE alertEnabled = true, used by the residential alert listeners and
  the saved-search cron (supports GOO-57 H-1 / H-2 follow-up work —
  eliminating the seq scan is the prerequisite for cursor batching).

Benchmarks (local PG16, synthetic data):

Property FTS with a selective term (50k rows, ~10 matching):
  with idx_property_fts:    3.97 ms (Bitmap Heap Scan, 338 buffers)
  without index:          242.56 ms (Parallel Seq Scan, 1784 buffers)
  → ~61x faster.

SavedSearch alert scan (100k rows, 5% alertEnabled, LIMIT 500 ORDER BY
createdAt DESC):
  with idx_savedsearch_alert_enabled:  0.48 ms (Index Scan Backward)
  without index:                       6.05 ms (Seq Scan + top-N sort)
  → ~12x faster, seq scan eliminated.

Hook-up verified: pnpm db:generate clean; raw migration applies via
prisma migrate deploy; post-migration \d confirms both indexes are
present with the expected definitions.

Refs: GOO-118, GOO-57
Co-Authored-By: Paperclip <noreply@paperclip.ing>

prisma/migrations/20260423130000_goo118_query_optimization_indexes/migration.sql

@@ -0,0 +1,30 @@
+-- GOO-118 — DB query optimization migration
+-- Source: GOO-57 audit (plan document: /GOO/issues/GOO-57#document-plan)
+--
+-- Changes:
+--   1. GIN expression index for Property full-text search (addresses M-3)
+--      Matches the exact expression used by
+--      apps/api/src/modules/search/infrastructure/services/search-query-builder.ts (FTS_COLUMNS).
+--
+--   2. Partial index on SavedSearch (createdAt) WHERE alertEnabled = true (supports H-1 / H-2)
+--      Listener / cron loads rows filtered by alertEnabled = true; partial index
+--      lets the query skip the seq scan and stays small (only ~alertEnabled rows).
+
+-- 1) GIN FTS index on Property
+CREATE INDEX IF NOT EXISTS "idx_property_fts"
+  ON "Property"
+  USING GIN (
+    to_tsvector(
+      'simple',
+      coalesce("title", '') || ' ' ||
+      coalesce("description", '') || ' ' ||
+      coalesce("address", '') || ' ' ||
+      coalesce("district", '') || ' ' ||
+      coalesce("city", '')
+    )
+  );
+
+-- 2) Partial index: only alert-enabled saved searches
+CREATE INDEX IF NOT EXISTS "idx_savedsearch_alert_enabled"
+  ON "SavedSearch" ("createdAt")
+  WHERE "alertEnabled" = true;