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/20260116092209_create_book_genres_table.sql

@@ -0,0 +1,13 @@
+-- Create book_genres junction table
+CREATE TABLE IF NOT EXISTS book_genres (
+    id SERIAL PRIMARY KEY,
+    genre INTEGER NOT NULL,
+    book INTEGER NOT NULL,
+    CONSTRAINT uc_book_genres_book_genre UNIQUE (book, genre),
+    CONSTRAINT fk_book_genres_genres_genre_id FOREIGN KEY (genre) REFERENCES genres(id) ON UPDATE CASCADE ON DELETE CASCADE,
+    CONSTRAINT fk_book_genres_books_book_id FOREIGN KEY (book) REFERENCES books(id) ON UPDATE CASCADE ON DELETE CASCADE
+);
+
+-- Create indexes for book_genres
+CREATE INDEX IF NOT EXISTS book_genres_genre ON book_genres (genre);
+CREATE INDEX IF NOT EXISTS book_genres_book ON book_genres (book);