Add timezone support to database schema and queries

- Add timezone column (VARCHAR(50) DEFAULT 'UTC') to users table
- Add default_timezone row to system_settings seed data
- Add idx_users_timezone index for user timezone lookups
- Add UpdateUserTimezone and GetSystemTimezone queries
- Regenerate sqlc code (models, querier, queries.sql.go)
- Reuse existing UpdateSystemSetting for system timezone updates
  instead of creating a redundant UpdateSystemTimezone query

internal/database/queries.sql.go

@@ -5408,6 +5408,17 @@ func (q *Queries) GetSystemSetting(ctx context.Context, settingKey string) (stri
 	return setting_value, err
 }
 
+const GetSystemTimezone = `-- name: GetSystemTimezone :one
+SELECT setting_value FROM system_settings WHERE setting_key = 'default_timezone'
+`
+
+func (q *Queries) GetSystemTimezone(ctx context.Context) (string, error) {
+	row := q.db.QueryRow(ctx, GetSystemTimezone)
+	var setting_value string
+	err := row.Scan(&setting_value)
+	return setting_value, err
+}
+
 const GetUniversalProgress = `-- name: GetUniversalProgress :one
 SELECT
     rp.id,
@@ -10256,7 +10267,7 @@ func (q *Queries) UpdateUniversalProgress(ctx context.Context, arg UpdateUnivers
 
 const UpdateUserMaxDevices = `-- name: UpdateUserMaxDevices :one
 UPDATE users SET max_devices = $2, updated_at = NOW() WHERE id = $1
-RETURNING id, email, username, password_hash, first_name, last_name, role, theme, max_devices, created_at, updated_at
+RETURNING id, email, username, password_hash, first_name, last_name, role, theme, max_devices, created_at, timezone, updated_at
 `
 
 type UpdateUserMaxDevicesParams struct {
@@ -10278,6 +10289,7 @@ func (q *Queries) UpdateUserMaxDevices(ctx context.Context, arg UpdateUserMaxDev
 		&i.Theme,
 		&i.MaxDevices,
 		&i.CreatedAt,
+		&i.Timezone,
 		&i.UpdatedAt,
 	)
 	return i, err
@@ -10341,6 +10353,20 @@ func (q *Queries) UpdateUserTheme(ctx context.Context, arg UpdateUserThemeParams
 	return err
 }
 
+const UpdateUserTimezone = `-- name: UpdateUserTimezone :exec
+UPDATE users SET timezone = $2, updated_at = NOW() WHERE id = $1
+`
+
+type UpdateUserTimezoneParams struct {
+	ID       pgtype.UUID `db:"id" json:"id"`
+	Timezone pgtype.Text `db:"timezone" json:"timezone"`
+}
+
+func (q *Queries) UpdateUserTimezone(ctx context.Context, arg UpdateUserTimezoneParams) error {
+	_, err := q.db.Exec(ctx, UpdateUserTimezone, arg.ID, arg.Timezone)
+	return err
+}
+
 const UpdateUsername = `-- name: UpdateUsername :exec
 UPDATE users SET username = $2, updated_at = NOW() WHERE id = $1
 `