feat(data): FTS5 entity search engine

Adds a SQLite FTS5 virtual table `entities_fts` indexing seven entity
types (projects, vendors, appliances, incidents, quotes, maintenance
items, service logs) and the Go engine to populate and query it.

- `entities_fts` virtual table with porter + unicode61 tokenization for
  stem-folded cross-language matching.
- `RebuildFTSIndex` rebuilds the index from scratch from the live
  entity tables; safe to call repeatedly.
- `populateEntitiesFTS` skips soft-deleted rows (including soft-deleted
  parents when JOINing) so the index never surfaces deleted data.
- `SearchEntities` runs a MATCH query, returns BM25-ranked results
  with a stable `entity_id` tiebreaker, capped at 20 rows.
- `EntitySummary` fetches a tri-state result (found / stale / missing)
  so callers can revalidate cached search hits before using them.
- `truncateField` clips indexed content by rune count to keep the FTS
  shadow table bounded without splitting UTF-8 sequences.

Test coverage: creation, rebuild, soft-delete exclusion (own rows
and parents), cross-entity search, stemming, graceful degradation on
corrupted schema, stale revalidation, Unicode truncation, tiebreaker
determinism.

This is the index and query layer only. Context-formatting helpers
and caller wiring land in follow-up PRs.

Refs #707.

Author: cpcloud