feat: implement lexical search functionality in RAG, including new SQL queries and service methods

db/migrations/0000018_rag_chunks_content_trgm.down.sql

@@ -0,0 +1 @@
+DROP INDEX IF EXISTS idx_rag_chunks_content_trgm;

db/migrations/0000018_rag_chunks_content_trgm.up.sql

@@ -0,0 +1,4 @@
+CREATE EXTENSION IF NOT EXISTS pg_trgm;
+
+CREATE INDEX IF NOT EXISTS idx_rag_chunks_content_trgm
+ON rag_chunks USING GIN (content gin_trgm_ops);