feat: add full-text search module

[Xaxxoo]

Summary

Closes #369

This PR adds the first usable full-text search module for users, messages, and conversations.

What changed

• Added SearchModule, service, controller, and indexed-document API.
• Added idempotent upsert and deletion operations for indexing workers.
• Added PostgreSQL websearch_to_tsquery search with ILIKE fallback, relevance ordering, highlighted snippets, resource filtering, and pagination.
• Added facet counts by resource type.
• Added normalized search result DTO behavior and Swagger-facing API routes.
• Added focused service tests for upserts, query constraints, pagination, result normalization, and facets.
• Added docs/SEARCH.md with indexing and query integration guidance.

Verification

• npm test -- --runInBand src/search/search.service.spec.ts
• npm run build:tsc
• git diff --check

All checks pass.