Add Elasticsearch as optional search backend for library catalog

[jakebromberg]

Context

The library catalog search currently uses PostgreSQL pg_trgm (trigram similarity) via the library_artist_view database view. While functional, pg_trgm has limitations: it doesn't support stemming, synonym expansion, or sophisticated relevance ranking. Elasticsearch provides Lucene-powered full-text search with fuzzy matching, relevance tuning, and better handling of partial matches.

Plan

Add Elasticsearch as an optional search backend with graceful degradation to pg_trgm when ES is unavailable. PostgreSQL remains the source of truth.

PR 1 (this issue): ES client, index management, search service, facade with fallback, unit tests, Docker Compose, healthcheck

New files (apps/backend/services/search/):

• elasticsearch.client.ts -- Singleton ES client. Returns null when ELASTICSEARCH_URL is unset.
• elasticsearch.indices.ts -- Index mapping and lifecycle (ensureLibraryIndex() called at startup).
• elasticsearch.search.ts -- ES query implementations matching existing function signatures.
• elasticsearch.sync.ts -- Stub interfaces for dual-write sync (PR 2).
• index.ts -- Facade: tries ES first, falls back to pg_trgm on error.

Modified files:

• library.service.ts -- Renames pg_trgm functions with pgTrgm prefix, re-exports facade versions under original names.
• app.ts -- Calls ensureLibraryIndex() at startup, expands /healthcheck with ES status.
• apps/backend/package.json -- Adds @elastic/elasticsearch ^8.17.0.
• dev_env/docker-compose.yml -- Adds ES services for dev and ci profiles.
• scripts/ci-env.sh -- Optionally starts ci-elasticsearch.
• .env.example and CLAUDE.md -- Documents new env vars and architecture.

Unit tests (tests/unit/services/search/):

• elasticsearch.client.test.ts -- Client enable/disable, singleton, auth config.
• elasticsearch.search.test.ts -- Query structure, result mapping, empty results.
• search.facade.test.ts -- ES routing, fallback on error, direct pg_trgm when disabled.

PR 2 (follow-up): Dual-write sync from library write paths, bulk reindex job, integration tests with real ES container

Feature Flag

ELASTICSEARCH_URL env var:

• Unset (default): pg_trgm only
• Set (e.g., http://localhost:9200): ES with automatic fallback to pg_trgm
• Instant rollback: unset the env var and restart