feat: persistent per-application config with GET/PUT endpoints

Add application_config table (V4 migration), repository, and REST
controller. GET /api/v1/config/{app} returns config, PUT saves and
pushes CONFIG_UPDATE to all LIVE agents via SSE. UI tracing toggle
now uses config API instead of direct SET_TRACED_PROCESSORS command.
Tracing store syncs with server config on load.

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

ui/src/api/queries/commands.ts

@@ -1,6 +1,51 @@
-import { useMutation } from '@tanstack/react-query'
+import { useMutation, useQuery, useQueryClient } from '@tanstack/react-query'
 import { api } from '../client'
 
+// ── Application Config ────────────────────────────────────────────────────
+
+export interface ApplicationConfig {
+  application: string
+  version: number
+  updatedAt?: string
+  engineLevel?: string
+  payloadCaptureMode?: string
+  metricsEnabled: boolean
+  samplingRate: number
+  tracedProcessors: Record<string, string>
+}
+
+export function useApplicationConfig(application: string | undefined) {
+  return useQuery({
+    queryKey: ['applicationConfig', application],
+    queryFn: async () => {
+      const res = await fetch(`/api/v1/config/${application}`)
+      if (!res.ok) throw new Error('Failed to fetch config')
+      return res.json() as Promise<ApplicationConfig>
+    },
+    enabled: !!application,
+  })
+}
+
+export function useUpdateApplicationConfig() {
+  const queryClient = useQueryClient()
+  return useMutation({
+    mutationFn: async (config: ApplicationConfig) => {
+      const res = await fetch(`/api/v1/config/${config.application}`, {
+        method: 'PUT',
+        headers: { 'Content-Type': 'application/json' },
+        body: JSON.stringify(config),
+      })
+      if (!res.ok) throw new Error('Failed to update config')
+      return res.json() as Promise<ApplicationConfig>
+    },
+    onSuccess: (saved) => {
+      queryClient.setQueryData(['applicationConfig', saved.application], saved)
+    },
+  })
+}
+
+// ── Generic Group Command (kept for non-config commands) ──────────────────
+
 interface SendGroupCommandParams {
   group: string
   type: string