docs(ble): add pairing guide, remove unused MC_BLE_PIN config

MC_BLE_PIN was non-functional — bleak in Docker cannot perform
interactive pairing (no BlueZ agent). Pairing must be done on
the host before starting mc-webui. Added comprehensive pairing
guide at docs/meshcore_bluetooth_pairing.md.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/config.py

@@ -33,8 +33,8 @@ class Config:
     MC_TCP_PORT = int(os.getenv('MC_TCP_PORT', '5555'))
 
     # v2: BLE connection (alternative to serial/TCP, for BLE companion devices)
+    # Device must be paired and trusted on host before starting (see docs/meshcore_bluetooth_pairing.md)
     MC_BLE_ADDRESS = os.getenv('MC_BLE_ADDRESS', '')  # BLE MAC address or device name filter
-    MC_BLE_PIN = os.getenv('MC_BLE_PIN', '')           # PIN for BLE pairing
 
     # v2: Backup
     MC_BACKUP_ENABLED = os.getenv('MC_BACKUP_ENABLED', 'true').lower() == 'true'

app/device_manager.py

@@ -189,7 +189,6 @@ class DeviceManager:
                 logger.info(f"Connecting via BLE: {self.config.MC_BLE_ADDRESS}")
                 self.mc = await MeshCore.create_ble(
                     address=self.config.MC_BLE_ADDRESS,
-                    pin=self.config.MC_BLE_PIN or None,
                     auto_reconnect=False,
                 )
             elif self.config.use_tcp: