fix: remove stale SQLite-era Beacon block in message_handler

[jlunder00]

Summary

• Removes the stale # Premium Beacon hook — review accumulated changes on anchor transition try/except block (~42 lines) from bot/message_handler.py.
• This block called should_trigger_beacon(str(_active_db), ...) and run_beacon(db_path=...), but after the Postgres migration both functions take asyncpg.Connection, not a db path string. Every invocation silently hit the except Exception handler.
• The live Beacon trigger is _do_beacon_check in tether_premium/bot/register.py and is unaffected.
• Also removes the now-dead _active_db = last_user_id assignment.

Test plan

• New test asserts the stale Beacon string is not present in message_handler source
• Full test suite passes: python -m pytest tests/ -x -q