feat(database): add user_library table and library access methods

Signed-off-by: Deluan <deluan@navidrome.org>

# Conflicts:
#	tests/mock_library_repo.go

db/migrations/20250106113337_add_user_library_table.go

@@ -0,0 +1,56 @@
+package migrations
+
+import (
+	"context"
+	"database/sql"
+
+	"github.com/pressly/goose/v3"
+)
+
+func init() {
+	goose.AddMigrationContext(upAddUserLibraryTable, downAddUserLibraryTable)
+}
+
+func upAddUserLibraryTable(ctx context.Context, tx *sql.Tx) error {
+	// Create user_library association table
+	_, err := tx.ExecContext(ctx, `
+		CREATE TABLE user_library (
+			user_id VARCHAR(255) NOT NULL,
+			library_id INTEGER NOT NULL,
+			PRIMARY KEY (user_id, library_id),
+			FOREIGN KEY (user_id) REFERENCES user(id) ON DELETE CASCADE,
+			FOREIGN KEY (library_id) REFERENCES library(id) ON DELETE CASCADE
+		);
+	`)
+	if err != nil {
+		return err
+	}
+
+	// Create indexes for performance
+	_, err = tx.ExecContext(ctx, `
+		CREATE INDEX idx_user_library_user_id ON user_library(user_id);
+		CREATE INDEX idx_user_library_library_id ON user_library(library_id);
+	`)
+	if err != nil {
+		return err
+	}
+
+	// Populate with existing users having access to library ID 1 (existing setup)
+	// Admin users get access to all libraries, regular users get access to library 1
+	_, err = tx.ExecContext(ctx, `
+		INSERT INTO user_library (user_id, library_id)
+		SELECT u.id, 1
+		FROM user u;
+	`)
+
+	return err
+}
+
+func downAddUserLibraryTable(ctx context.Context, tx *sql.Tx) error {
+	_, err := tx.ExecContext(ctx, `
+		DROP INDEX IF EXISTS idx_user_library_library_id;
+		DROP INDEX IF EXISTS idx_user_library_user_id;
+		DROP TABLE IF EXISTS user_library;
+	`)
+	return err
+}