feat: implement hybrid document search combining Quickwit results with PostgreSQL pg_trgm title search, supported by a new index.

backend/migrations/202511300100_add_title_trgm_index/down.sql

@@ -0,0 +1 @@
+DROP INDEX IF EXISTS tenant.idx_documents_title_trgm;

backend/migrations/202511300100_add_title_trgm_index/up.sql

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+CREATE EXTENSION IF NOT EXISTS pg_trgm;
+
+CREATE INDEX idx_documents_title_trgm
+    ON tenant.documents
+    USING gin (title gin_trgm_ops)
+    WHERE deleted_at IS NULL;