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/queries.sql

@@ -300,6 +300,12 @@ RETURNING *;
 UPDATE users SET role = $2, updated_at = NOW() WHERE id = $1
 RETURNING id, email, username, role;
 
+-- name: UpdateUserTimezone :exec
+UPDATE users SET timezone = $2, updated_at = NOW() WHERE id = $1;
+
+-- name: GetSystemTimezone :one
+SELECT setting_value FROM system_settings WHERE setting_key = 'default_timezone';
+
 -- name: CountUserDevices :one
 SELECT COUNT(*) FROM devices WHERE user_id = $1;