fix(poller): fall back to generic-snmp when device has no profile assigned

SNMP devices added without a profile (e.g., via simplified add flow)
were failing with "no SNMP profile assigned". Now falls back to the
generic-snmp profile which collects standard MIB-II metrics.

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

poller/internal/snmp/profiles.go

@@ -154,6 +154,13 @@ func (c *ProfileCache) Get(profileID string) *CompiledProfile {
 	return c.profiles[profileID]
 }
 
+// GetGenericID returns the profile ID of the generic-snmp fallback profile.
+func (c *ProfileCache) GetGenericID() string {
+	c.mu.RLock()
+	defer c.mu.RUnlock()
+	return c.genericID
+}
+
 // MatchSysObjectID finds the best profile for a device's sysObjectID value
 // using longest-prefix matching. Returns the generic-snmp profile ID if
 // no vendor-specific prefix matches.