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

ui/src/pages/ExchangeDetail/ExchangeDetail.tsx

@@ -1,4 +1,4 @@
-import { useState, useMemo, useCallback } from 'react'
+import { useState, useMemo, useCallback, useEffect } from 'react'
 import { useParams, useNavigate } from 'react-router'
 import {
   Badge, StatusDot, MonoText, CodeBlock, InfoCallout,
@@ -10,7 +10,7 @@ import { useCorrelationChain } from '../../api/queries/correlation'
 import { useDiagramLayout } from '../../api/queries/diagrams'
 import { mapDiagramToRouteNodes } from '../../utils/diagram-mapping'
 import { useTracingStore } from '../../stores/tracing-store'
-import { useSendGroupCommand } from '../../api/queries/commands'
+import { useApplicationConfig, useUpdateApplicationConfig } from '../../api/queries/commands'
 import styles from './ExchangeDetail.module.css'
 
 // ── Helpers ──────────────────────────────────────────────────────────────────
@@ -187,27 +187,35 @@ export default function ExchangeDetail() {
   // ── Tracing toggle ──────────────────────────────────────────────────────
   const { toast } = useToast()
   const tracingStore = useTracingStore()
-  const sendCommand = useSendGroupCommand()
   const app = detail?.applicationName ?? ''
+  const { data: appConfig } = useApplicationConfig(app || undefined)
+  const updateConfig = useUpdateApplicationConfig()
+
+  // Sync tracing store with server config
+  useEffect(() => {
+    if (appConfig?.tracedProcessors && app) {
+      tracingStore.syncFromServer(app, appConfig.tracedProcessors)
+    }
+  }, [appConfig, app])
 
   const handleToggleTracing = useCallback((processorId: string) => {
-    if (!processorId || !detail?.applicationName) return
+    if (!processorId || !detail?.applicationName || !appConfig) return
     const newMap = tracingStore.toggleProcessor(app, processorId)
-    sendCommand.mutate({
-      group: detail.applicationName,
-      type: 'set-traced-processors',
-      payload: { processors: newMap },
-    }, {
-      onSuccess: (data) => {
+    const updatedConfig = {
+      ...appConfig,
+      tracedProcessors: { ...newMap },
+    }
+    updateConfig.mutate(updatedConfig, {
+      onSuccess: (saved) => {
         const action = processorId in newMap ? 'enabled' : 'disabled'
-        toast({ title: `Tracing ${action}`, description: `${processorId} — sent to ${data?.targetCount ?? 0} agent(s)`, variant: 'success' })
+        toast({ title: `Tracing ${action}`, description: `${processorId} — config v${saved.version}`, variant: 'success' })
       },
       onError: () => {
         tracingStore.toggleProcessor(app, processorId)
-        toast({ title: 'Command failed', description: 'Could not send tracing command', variant: 'error' })
+        toast({ title: 'Config update failed', description: 'Could not save configuration', variant: 'error' })
       },
     })
-  }, [detail, app, tracingStore, sendCommand, toast])
+  }, [detail, app, appConfig, tracingStore, updateConfig, toast])
 
   // Correlation chain
   const correlatedExchanges = useMemo(() => {
@@ -372,7 +380,7 @@ export default function ExchangeDetail() {
                   return [{
                     label: tracingStore.isTraced(app, pid) ? 'Disable Tracing' : 'Enable Tracing',
                     onClick: () => handleToggleTracing(pid),
-                    disabled: sendCommand.isPending,
+                    disabled: updateConfig.isPending,
                   }]
                 }}
               />
@@ -391,7 +399,7 @@ export default function ExchangeDetail() {
                   return [{
                     label: tracingStore.isTraced(app, pid) ? 'Disable Tracing' : 'Enable Tracing',
                     onClick: () => handleToggleTracing(pid),
-                    disabled: sendCommand.isPending,
+                    disabled: updateConfig.isPending,
                   }]
                 }}
               />

ui/src/stores/tracing-store.ts

@@ -8,6 +8,8 @@ interface TracingState {
   isTraced: (app: string, processorId: string) => boolean
   /** Toggle processor tracing (BOTH on, remove on off). Returns the full map for the app. */
   toggleProcessor: (app: string, processorId: string) => Record<string, CaptureMode>
+  /** Sync store with server-side config (called when config is fetched). */
+  syncFromServer: (app: string, tracedProcessors: Record<string, string>) => void
 }
 
 export const useTracingStore = create<TracingState>((set, get) => ({
@@ -25,4 +27,14 @@ export const useTracingStore = create<TracingState>((set, get) => ({
     }))
     return current
   },
+
+  syncFromServer: (app, serverMap) => {
+    const mapped: Record<string, CaptureMode> = {}
+    for (const [k, v] of Object.entries(serverMap)) {
+      mapped[k] = v as CaptureMode
+    }
+    set((state) => ({
+      tracedProcessors: { ...state.tracedProcessors, [app]: mapped },
+    }))
+  },
 }))