Change default timezone from Europe/Warsaw to UTC

[adrunkhuman]

Problem

The default timezone is hardcoded to Europe/Warsaw in typer_bot/utils/timezone.py. This is opinionated and confusing for non-European users.

Impact

• Non-European users see incorrect times by default
• Hardcoded regional preference reduces global accessibility

Proposed Solution

Default to UTC for maximum compatibility. Users can still override via TZ environment variable.

Acceptance Criteria

• Default timezone changed to UTC
• .env.example updated with TZ=UTC
• No breaking change (existing TZ overrides continue working)

Related

• Feature: Configurable server/channel/role setup #52 (Configurable setup) - mentions per-server timezone override
• Feature: Proper season/competition management #51 (Season management) - international competitions