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/20260116092205_create_authors_table.sql

@@ -0,0 +1,15 @@
+-- Create authors table
+CREATE TABLE IF NOT EXISTS authors (
+    id SERIAL PRIMARY KEY,
+    source SMALLINT NOT NULL,
+    remote_id INTEGER NOT NULL,
+    first_name VARCHAR(256) NOT NULL,
+    last_name VARCHAR(256) NOT NULL,
+    middle_name VARCHAR(256),
+    CONSTRAINT uc_authors_source_remote_id UNIQUE (source, remote_id),
+    CONSTRAINT fk_authors_sources_id_source FOREIGN KEY (source) REFERENCES sources(id)
+);
+
+-- Create trigram indexes for author search
+CREATE INDEX IF NOT EXISTS tgrm_authors_lf ON authors USING gin ((last_name || ' ' || first_name) gin_trgm_ops);
+CREATE INDEX IF NOT EXISTS tgrm_authors_lfm ON authors USING gin ((last_name || ' ' || first_name || ' ' || middle_name) gin_trgm_ops);