feat(18-05): wire SNMPCollector into Scheduler and main.go

- Add RegisterCollector method to Scheduler for external collector registration
- Initialize ProfileCache with 5-min refresh in main.go
- Initialize CounterCache using existing Redis client
- Create and register SNMPCollector as "snmp" in scheduler
- Start DiscoveryResponder for NATS SNMP device auto-detection
- Existing RouterOS polling path completely unaffected

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

poller/internal/poller/scheduler.go

@@ -87,12 +87,19 @@ func NewScheduler(
 		activeDevices:   make(map[string]*deviceState),
 	}
 
-	// Register built-in collectors. Future device types (SNMP) register here.
+	// Register built-in collectors.
 	s.collectors["routeros"] = NewRouterOSCollector(locker, credentialCache, connTimeout, cmdTimeout, lockTTL)
 
 	return s
 }
 
+// RegisterCollector adds a named Collector to the scheduler's dispatch map.
+// This allows external packages (e.g., SNMP) to register collectors without
+// modifying NewScheduler's parameter list.
+func (s *Scheduler) RegisterCollector(name string, c Collector) {
+	s.collectors[name] = c
+}
+
 // Run is the main scheduler loop. It:
 //  1. Fetches devices from the database.
 //  2. Starts goroutines for newly-discovered devices.