feat(data): FTS5 entity search engine (#960)

## Summary

- SQLite FTS5 virtual table `entities_fts` indexing seven entity types
(projects, vendors, appliances, incidents, quotes, maintenance items,
service logs) with porter + unicode61 tokenization.
- `RebuildFTSIndex` populates the index from the live entity tables.
Safe to call repeatedly; skips soft-deleted rows (own rows and
soft-deleted parents in JOINs).
- `SearchEntities` runs a MATCH query and 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.
- `truncateField` clips indexed content by rune count without splitting
UTF-8 sequences.

This PR is the index and query layer only. No callers are wired up yet —
triggers, query hardening, and the eval subcommand land in follow-up
PRs.

Refs #707

Author: cpcloud