fix(20): add SNMP profiles settings link and device_count to profile list

Adds SNMP Device Profiles card to SettingsPage for discoverability.
Adds device_count correlated subquery to profile list SQL and schema
field so the frontend profile cards show accurate device counts.

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

backend/app/schemas/snmp_profile.py

@@ -75,6 +75,7 @@ class SNMPProfileResponse(BaseModel):
     vendor: Optional[str] = None
     category: Optional[str] = None
     is_system: bool
+    device_count: int = 0
     created_at: datetime
     updated_at: datetime