feat(constants): add Topics.SYSTEM_ARM_REQUEST

Topic for web-originated arm/disarm requests that the event processor
will subscribe to and dispatch to ArmStateFSM.transition. Part of the
PIN unification work (issue #2).

vigilar/constants.py

@@ -210,6 +210,8 @@ class Topics:
     SYSTEM_ALERT = "vigilar/system/alert"
     SYSTEM_SHUTDOWN = "vigilar/system/shutdown"
     SYSTEM_RULES_UPDATED = "vigilar/system/rules_updated"
+    # Web-to-FSM arm/disarm request (FSM verifies the PIN and transitions)
+    SYSTEM_ARM_REQUEST = "vigilar/system/arm_request"
 
     # Wildcard subscriptions
     ALL = "vigilar/#"