Make fallback timezone configurable

[animaworks-dev-team]

Summary

core/time_utils.py has a hardcoded _FALLBACK_TZ = ZoneInfo("Asia/Tokyo"). This should be configurable for non-Japanese deployments.

What to do

1. Add a config option (env var ANIMAWORKS_TIMEZONE or config.json field) for the default timezone
2. Fall back to Asia/Tokyo if not set (backward compatible)
3. Update core/time_utils.py to read from the config

How to verify

ANIMAWORKS_TIMEZONE=America/New_York uv run animaworks start
# Timestamps in the UI and logs should reflect the configured timezone

Notes

• Use zoneinfo.ZoneInfo (stdlib) — no new dependencies needed
• Keep Asia/Tokyo as the default to avoid breaking existing setups