Refactor threads to use daemon threads and Event-based timing

[hemna]

Summary

This PR refactors all APRSD thread classes to:

• Use daemon threads by default (with explicit non-daemon for critical I/O threads)
• Replace counter-based sleep patterns with threading.Event() for interruptible waits
• Enable faster, cleaner shutdown

Changes

Base Class (aprsd/threads/aprsd.py)

• Add daemon = True class attribute (subclasses override to False)
• Add period = 1 class attribute for wait interval
• Replace thread_stop boolean with _shutdown_event (threading.Event)
• Add wait() method for interruptible sleeps
• Add join_non_daemon() to APRSDThreadList for graceful shutdown

Non-Daemon Threads (3)

These threads are marked daemon = False for graceful shutdown:

• PacketSendSchedulerThread - manages packet send queue
• AckSendSchedulerThread - manages ACK send queue
• APRSDStatsStoreThread - writes stats to disk

Thread Migrations

All threads migrated from counter-based timing to period-based:

• KeepAliveThread - period=60
• APRSDStatsStoreThread - period=10
• APRSDPushStatsThread - period from config
• StatsLogThread - period=10
• APRSDRXThread - interruptible error recovery
• APRSDFilterThread - queue timeout as wait
• BeaconSendThread - period from config
• APRSRegistryThread - period from config

Signal Handler

• Replace time.sleep(1.5) with join_non_daemon(timeout=5.0)

Benefits

• Faster shutdown: Threads respond immediately to shutdown signal instead of waiting for time.sleep() to finish
• Cleaner code: No more counter math (loop_count % 60 == 0)
• Proper daemon semantics: Program can exit cleanly; critical threads get graceful shutdown

Testing

• All 679 tests pass
• Linting passes (ruff check + format)

Breaking Changes

• thread_stop boolean replaced with _shutdown_event
• Code checking thread.thread_stop directly must use thread._shutdown_event.is_set()