Search upgrade: Postgres full-text (tsvector) and pgvector semantic search

Context: Search is ILIKE-based today (src/lib/mcp/tools.ts). pgvector is enabled but unused; pg_trgm powers duplicate suggestions only.

Scope:

• Add tsvector columns and GIN indexes for posts, docs, and changelog; rank results.
• Add an embeddings pipeline and pgvector index for semantic search and better duplicate detection.
• Blend lexical and semantic results in the search UI and the MCP search tool.

Done when: search returns ranked, relevant results across posts, docs, and changelog with semantic recall.