feat(CF-1315): Hybrid search with tsvector + RRF

Add PostgreSQL full-text search alongside pgvector for exact matches
on Jira keys, error messages, file paths. Merge results with
Reciprocal Rank Fusion. Default mode: hybrid, with graceful
degradation to keyword-only when embeddings unavailable.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

migrations/034_hybrid_search.sql

@@ -0,0 +1,53 @@
+-- CF-1315: Hybrid search - tsvector columns, GIN indexes, triggers
+
+-- 1. Add search_vector columns
+ALTER TABLE project_archives ADD COLUMN IF NOT EXISTS search_vector tsvector;
+ALTER TABLE memories ADD COLUMN IF NOT EXISTS search_vector tsvector;
+ALTER TABLE sessions ADD COLUMN IF NOT EXISTS search_vector tsvector;
+
+-- 2. GIN indexes for fast full-text search
+CREATE INDEX IF NOT EXISTS idx_archives_search_vector ON project_archives USING gin(search_vector);
+CREATE INDEX IF NOT EXISTS idx_memories_search_vector ON memories USING gin(search_vector);
+CREATE INDEX IF NOT EXISTS idx_sessions_search_vector ON sessions USING gin(search_vector);
+
+-- 3. Triggers to auto-populate search_vector on INSERT/UPDATE
+CREATE OR REPLACE FUNCTION update_archives_search_vector() RETURNS TRIGGER AS $$
+BEGIN
+  NEW.search_vector := to_tsvector('english', coalesce(NEW.title, '') || ' ' || coalesce(NEW.content, ''));
+  RETURN NEW;
+END;
+$$ LANGUAGE plpgsql;
+
+CREATE OR REPLACE FUNCTION update_memories_search_vector() RETURNS TRIGGER AS $$
+BEGIN
+  NEW.search_vector := to_tsvector('english', coalesce(NEW.title, '') || ' ' || coalesce(NEW.content, ''));
+  RETURN NEW;
+END;
+$$ LANGUAGE plpgsql;
+
+CREATE OR REPLACE FUNCTION update_sessions_search_vector() RETURNS TRIGGER AS $$
+BEGIN
+  NEW.search_vector := to_tsvector('english', coalesce(NEW.summary, ''));
+  RETURN NEW;
+END;
+$$ LANGUAGE plpgsql;
+
+DROP TRIGGER IF EXISTS trg_archives_search_vector ON project_archives;
+CREATE TRIGGER trg_archives_search_vector
+  BEFORE INSERT OR UPDATE OF title, content ON project_archives
+  FOR EACH ROW EXECUTE FUNCTION update_archives_search_vector();
+
+DROP TRIGGER IF EXISTS trg_memories_search_vector ON memories;
+CREATE TRIGGER trg_memories_search_vector
+  BEFORE INSERT OR UPDATE OF title, content ON memories
+  FOR EACH ROW EXECUTE FUNCTION update_memories_search_vector();
+
+DROP TRIGGER IF EXISTS trg_sessions_search_vector ON sessions;
+CREATE TRIGGER trg_sessions_search_vector
+  BEFORE INSERT OR UPDATE OF summary ON sessions
+  FOR EACH ROW EXECUTE FUNCTION update_sessions_search_vector();
+
+-- 4. Backfill existing rows (no-op if tables empty, safe to re-run)
+UPDATE project_archives SET search_vector = to_tsvector('english', coalesce(title, '') || ' ' || coalesce(content, '')) WHERE search_vector IS NULL;
+UPDATE memories SET search_vector = to_tsvector('english', coalesce(title, '') || ' ' || coalesce(content, '')) WHERE search_vector IS NULL;
+UPDATE sessions SET search_vector = to_tsvector('english', coalesce(summary, '')) WHERE search_vector IS NULL AND summary IS NOT NULL;