PostgresBackend.Migrate hand-wrote the tsvector column, trigger, and GIN
index that migration 000001 already creates, with a drift (namespace
DEFAULT '' vs 'default'), and it created the pg_trgm extension + trigram
indexes that no production path ever called — so fuzzy search ran without
its indexes in production.

- Move pg_trgm + the trigram indexes into a new migration (000006). A DO
  block attempts CREATE EXTENSION and degrades gracefully (indexes only
  created when the extension is present) so a missing privilege does not
  fail startup. Bump RequiredSchemaVersion to 6.
- Reduce PostgresBackend.Migrate to running the versioned migrations, so
  there is one schema definition and it cannot drift.
- Point the Postgres search tests at the real migrations (reset schema +
  RunMigrations) instead of AutoMigrate + backend DDL, and adjust the
  down-migration step counts and required-version log assertions for v6.

Verified against a live Postgres: migrations create pg_trgm, both trigram
indexes, and namespace DEFAULT 'default'.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>