refactor: fix FTS search scaling

Previously ftsSearch did:
- SELECT id FROM codebases (1 query)
- N * SELECT from sqlite_master to check if each FTS table exists
- M individual SELECT from chunks per FTS result row

Now:
- One SELECT from sqlite_master GLOB 'fts_*' discovers all tables
- JOIN chunks inline in the FTS query eliminates per-row lookups

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>