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/20260116092208_create_book_authors_table.sql

@@ -0,0 +1,13 @@
+-- Create book_authors junction table
+CREATE TABLE IF NOT EXISTS book_authors (
+    id SERIAL PRIMARY KEY,
+    author INTEGER NOT NULL,
+    book INTEGER NOT NULL,
+    CONSTRAINT uc_book_authors_book_author UNIQUE (book, author),
+    CONSTRAINT fk_book_authors_authors_author_id FOREIGN KEY (author) REFERENCES authors(id) ON UPDATE CASCADE ON DELETE CASCADE,
+    CONSTRAINT fk_book_authors_books_book_id FOREIGN KEY (book) REFERENCES books(id) ON UPDATE CASCADE ON DELETE CASCADE
+);
+
+-- Create indexes for book_authors
+CREATE INDEX IF NOT EXISTS book_authors_author ON book_authors (author);
+CREATE INDEX IF NOT EXISTS book_authors_book ON book_authors (book);