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/20260116092206_create_sequences_table.sql

@@ -0,0 +1,9 @@
+-- Create sequences table
+CREATE TABLE IF NOT EXISTS sequences (
+    id SERIAL PRIMARY KEY,
+    source SMALLINT NOT NULL,
+    remote_id INTEGER NOT NULL,
+    name VARCHAR(256) NOT NULL,
+    CONSTRAINT uc_sequences_source_remote_id UNIQUE (source, remote_id),
+    CONSTRAINT fk_sequences_sources_id_source FOREIGN KEY (source) REFERENCES sources(id)
+);