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/20260116092203_create_sources_table.sql

@@ -0,0 +1,5 @@
+-- Create sources table
+CREATE TABLE IF NOT EXISTS sources (
+    id SMALLSERIAL PRIMARY KEY,
+    name VARCHAR(32) NOT NULL UNIQUE
+);