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/models.go

@@ -507,5 +507,6 @@ type Users struct {
 	Theme        pgtype.Text        `db:"theme" json:"theme"`
 	MaxDevices   pgtype.Int4        `db:"max_devices" json:"max_devices"`
 	CreatedAt    pgtype.Timestamptz `db:"created_at" json:"created_at"`
+	Timezone     pgtype.Text        `db:"timezone" json:"timezone"`
 	UpdatedAt    pgtype.Timestamptz `db:"updated_at" json:"updated_at"`
 }