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/20260116092212_create_author_annotations_table.sql

@@ -0,0 +1,12 @@
+-- Create author_annotations table
+CREATE TABLE IF NOT EXISTS author_annotations (
+    id SERIAL PRIMARY KEY,
+    author INTEGER NOT NULL UNIQUE,
+    title VARCHAR(256) NOT NULL,
+    text TEXT NOT NULL,
+    file VARCHAR(256),
+    CONSTRAINT fk_author_annotations_authors_id_author FOREIGN KEY (author) REFERENCES authors(id)
+);
+
+-- Create index for author_annotations
+CREATE INDEX IF NOT EXISTS author_annotation_author_id ON author_annotations (author);