feat(search): add matchField and PostgreSQL tsvector full-text index

[waterbro-8]

Summary

Follow-up to #68 / PR #69.

Phase 1 of #68 asked for a contentSearch tsvector + GIN index, websearch_to_tsquery(), and a matchField indicator (title | content | both) so host UIs can highlight where a hit was found. PR #69 ships title OR content contains plus after/before/sort, which unblocks the host-app search miss, but not the indexed retrieval or match provenance.

Expected

• Persist a plain-text extraction of TipTap content (or equivalent) and index it.
• GET /api/v1/documents?query= and GET /api/doc?keyword= report matchField.
• Keep the current contains fallback for SQLite / unmigrated environments.

Out of scope

• Host-app query syntax (type:json after:…) — that belongs in RoleWeave / the embedding app.
• MIME type and size filters (Phase 3 of Upgrade document search to multi-dimensional indexed retrieval (content, time, metadata) #68).