fix: make rf_mapper scan state thread-safe and harden sleep/shutdown

[dolepee]

Problem

RFmapper updates scan state from a background thread and reads it from the run loop without synchronization. This can produce inconsistent reads and race conditions.

There are also two runtime reliability issues:

• self.sleep(1) is called even though RFmapper has no sleep() method
• shutdown waits with an unconditional sleep before join()

Related issue: #1771

Changes

• add threading.Lock() (scan_lock) for shared scan state
• guard scan_idx, scan_results, and scan_last_sent reads/writes with the lock
• replace deprecated logging.warn with logging.warning
• remove unused self.seen_names
• replace self.sleep(1) with time.sleep(1.0)
• replace stop-time sleep + join with self.thread.join(timeout=1.0)

Validation

• python3 -m py_compile src/backgrounds/plugins/rf_mapper.py