feat: add configurable flow retention settings with super admin controls and pagination for workload flows

Add RuntimeSettingsRead/RuntimeSettingsUpdate schemas with flow_retention_hours field (1-8760 hours), implement runtime_setting helper to initialize/fetch runtime system setting with 24h default, add require_super_admin guard to validate * permission, update agent_heartbeat to use configurable retention_cutoff from runtime settings instead of hardcoded 24h, replace /settings GET endpoint to return RuntimeSettingsRead with flow_retention_hours,

backend/app/schemas/domain.py

@@ -168,6 +168,17 @@ class FirewallApplyRequest(BaseModel):
     dry_run: bool = True
 
 
+class RuntimeSettingsRead(BaseModel):
+    product: str = "NexaFabric"
+    firewall_apply_requires_preview: bool = True
+    agent_optional: bool = True
+    flow_retention_hours: int = 24
+
+
+class RuntimeSettingsUpdate(BaseModel):
+    flow_retention_hours: int = Field(ge=1, le=8760)
+
+
 class ClusterRead(OrmModel):
     id: str
     name: str