Remove timezone from User model and pass as request parameter

Timezone is now sent from the client on each request that needs it,
instead of storing it in the database. This simplifies the model and
allows timezone to be dynamic (useful for traveling users).

Changes:
- Remove timezone field from User entity
- Remove timezone from user registration
- GET /habits/today now requires ?timezone= query param
- Add migration to drop timezone column from database
- Update related tests

internal/infrastructure/persistence/sqlite/migrations.go

@@ -25,6 +25,10 @@ func RunMigrations(db *sql.DB) error {
 		return err
 	}
 
+	if err := removeTimezoneColumn(db); err != nil {
+		return err
+	}
+
 	return nil
 }
 
@@ -54,6 +58,23 @@ func addEmailVerificationColumns(db *sql.DB) error {
 	return nil
 }
 
+func removeTimezoneColumn(db *sql.DB) error {
+	exists, err := columnExists(db, "users", "timezone")
+	if err != nil {
+		return err
+	}
+
+	if !exists {
+		return nil
+	}
+
+	if _, err := db.Exec("ALTER TABLE users DROP COLUMN timezone"); err != nil {
+		return fmt.Errorf("failed to drop timezone column: %w", err)
+	}
+
+	return nil
+}
+
 func columnExists(db *sql.DB, table, column string) (bool, error) {
 	query := fmt.Sprintf("SELECT COUNT(*) FROM pragma_table_info('%s') WHERE name = ?", table)
 	var count int
@@ -69,7 +90,6 @@ CREATE TABLE IF NOT EXISTS users (
 	id TEXT PRIMARY KEY,
 	email TEXT UNIQUE NOT NULL,
 	password_hash TEXT NOT NULL,
-	timezone TEXT DEFAULT 'UTC',
 	created_at DATETIME DEFAULT CURRENT_TIMESTAMP,
 	updated_at DATETIME DEFAULT CURRENT_TIMESTAMP
 );