New search module: a message index (Meilisearch, MEILI_URL-gated; no-op when unset),
a SearchProjector that indexes on message.sent, and a SearchService whose permission
fence runs server-side — a query only touches the caller's own scope AND the threads
they currently belong to (resolved live from participant rows, so membership changes
reflect immediately). Every conversation kind (chat/mail/sms) is a message with a TEXT
part, so all are searchable through one index. POST /v1/search + /reindex (backfill).
5 fence tests green.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
