fix: restore default admin password and add reset flag

Restored 'admin123' as the default password in db init and added a -reset-admin flag to main.go.

internal/db/db.go

@@ -159,7 +159,7 @@ func (db *DB) RunMigrations() error {
 	}
 
 	if count == 0 {
-		hash, err := bcrypt.GenerateFromPassword([]byte("admin"), 12)
+		hash, err := bcrypt.GenerateFromPassword([]byte("admin123"), 12)
 		if err != nil {
 			return fmt.Errorf("failed to hash default password: %w", err)
 		}
@@ -167,7 +167,7 @@ func (db *DB) RunMigrations() error {
 		if err != nil {
 			return fmt.Errorf("failed to insert default admin: %w", err)
 		}
-		log.Println("Created default admin user with password 'admin' (must change on first login)")
+		log.Println("Created default admin user with password 'admin123' (must change on first login)")
 	}
 
 	// Default client