Add DB migrations and run them on startup

Enable sqlx "migrate" feature and add SQL migrations to create the
database schema: pg_trgm extension, sources, genres, authors, sequences,
books, junction tables, annotations, and supporting indexes

migrations/20260116092202_create_extensions.sql

@@ -0,0 +1,2 @@
+-- Create pg_trgm extension for trigram-based text search
+CREATE EXTENSION IF NOT EXISTS pg_trgm;