feat(phase8.1-8.2): dynamic worker concurrency via worker_configs

- Migration 054: worker_configs table (queue_name PK, max/min_concurrency,
  enabled, updated_at); seeds step_processing(8/2), thumbnail_rendering(1/1),
  ai_validation(4/1)
- WorkerConfig SQLAlchemy model
- apply_worker_concurrency beat task: reads enabled configs, broadcasts
  pool_grow to all Celery workers every 5min
- GET/PUT /api/worker/configs (admin): list + update per-queue concurrency
- docker-compose.yml: worker uses --autoscale=${MAX_CONCURRENCY:-8},${MIN_CONCURRENCY:-2};
  render-worker uses --autoscale=1,1 --concurrency=1
- WorkerManagement.tsx: "Concurrency Settings" section with +/- steppers
  and Save button per queue

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/pages/WorkerManagement.tsx

@@ -1,13 +1,16 @@
 import { useState } from 'react'
 import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query'
 import { toast } from 'sonner'
-import { RefreshCw, ChevronDown, ChevronRight, Cpu, Layers, Minus, Plus } from 'lucide-react'
+import { RefreshCw, ChevronDown, ChevronRight, Cpu, Layers, Minus, Plus, Settings2 } from 'lucide-react'
 import {
   getCeleryWorkers,
   getQueueStatus,
   scaleWorkers,
+  getWorkerConfigs,
+  updateWorkerConfig,
   type CeleryWorker,
   type ScaleRequest,
+  type WorkerConfig,
 } from '../api/worker'
 
 // ---------------------------------------------------------------------------
@@ -161,6 +164,93 @@ function QueueDepthRow({ queue, depth }: { queue: string; depth: number }) {
   )
 }
 
+// ---------------------------------------------------------------------------
+// Concurrency config row
+// ---------------------------------------------------------------------------
+
+function ConcurrencyConfigRow({ config }: { config: WorkerConfig }) {
+  const qc = useQueryClient()
+  const [minVal, setMinVal] = useState(config.min_concurrency)
+  const [maxVal, setMaxVal] = useState(config.max_concurrency)
+
+  const saveMut = useMutation({
+    mutationFn: () =>
+      updateWorkerConfig(config.queue_name, {
+        min_concurrency: minVal,
+        max_concurrency: maxVal,
+      }),
+    onSuccess: () => {
+      toast.success(`Saved concurrency for ${config.queue_name}`)
+      qc.invalidateQueries({ queryKey: ['worker-configs'] })
+    },
+    onError: (e: unknown) => {
+      const detail = (e as { response?: { data?: { detail?: string } } })?.response?.data?.detail
+      toast.error(detail ?? `Failed to save ${config.queue_name}`)
+    },
+  })
+
+  const isDirty = minVal !== config.min_concurrency || maxVal !== config.max_concurrency
+
+  return (
+    <div className="rounded-xl border border-border-default p-4 flex items-center justify-between gap-4 flex-wrap">
+      <div className="min-w-0">
+        <p className="text-sm font-medium text-content font-mono">{config.queue_name}</p>
+        <p className="text-xs text-content-muted mt-0.5">
+          {config.enabled ? 'enabled' : 'disabled'} · updated{' '}
+          {new Date(config.updated_at).toLocaleString()}
+        </p>
+      </div>
+      <div className="flex items-center gap-6 shrink-0">
+        {/* Min concurrency */}
+        <div className="flex flex-col items-center gap-1">
+          <span className="text-xs text-content-muted">Min</span>
+          <div className="flex items-center gap-1">
+            <button
+              onClick={() => setMinVal((v) => Math.max(1, v - 1))}
+              className="p-1 rounded-md bg-surface-muted hover:bg-surface-hover text-content transition-colors"
+            >
+              <Minus size={12} />
+            </button>
+            <span className="w-6 text-center text-sm font-semibold text-content">{minVal}</span>
+            <button
+              onClick={() => setMinVal((v) => Math.min(maxVal, v + 1))}
+              className="p-1 rounded-md bg-surface-muted hover:bg-surface-hover text-content transition-colors"
+            >
+              <Plus size={12} />
+            </button>
+          </div>
+        </div>
+        {/* Max concurrency */}
+        <div className="flex flex-col items-center gap-1">
+          <span className="text-xs text-content-muted">Max</span>
+          <div className="flex items-center gap-1">
+            <button
+              onClick={() => setMaxVal((v) => Math.max(minVal, v - 1))}
+              className="p-1 rounded-md bg-surface-muted hover:bg-surface-hover text-content transition-colors"
+            >
+              <Minus size={12} />
+            </button>
+            <span className="w-6 text-center text-sm font-semibold text-content">{maxVal}</span>
+            <button
+              onClick={() => setMaxVal((v) => Math.min(64, v + 1))}
+              className="p-1 rounded-md bg-surface-muted hover:bg-surface-hover text-content transition-colors"
+            >
+              <Plus size={12} />
+            </button>
+          </div>
+        </div>
+        <button
+          onClick={() => saveMut.mutate()}
+          disabled={saveMut.isPending || !isDirty}
+          className={`btn-primary text-xs px-3 py-1.5 ${!isDirty ? 'opacity-50 cursor-not-allowed' : ''}`}
+        >
+          {saveMut.isPending ? 'Saving…' : 'Save'}
+        </button>
+      </div>
+    </div>
+  )
+}
+
 // ---------------------------------------------------------------------------
 // Main page
 // ---------------------------------------------------------------------------
@@ -180,9 +270,15 @@ export default function WorkerManagement() {
     refetchInterval: 5_000,
   })
 
+  const { data: workerConfigs, isLoading: configsLoading } = useQuery({
+    queryKey: ['worker-configs'],
+    queryFn: getWorkerConfigs,
+  })
+
   function refresh() {
     qc.invalidateQueries({ queryKey: ['celery-workers'] })
     qc.invalidateQueries({ queryKey: ['queue-status'] })
+    qc.invalidateQueries({ queryKey: ['worker-configs'] })
   }
 
   const workers = workerData?.workers ?? []
@@ -263,6 +359,33 @@ export default function WorkerManagement() {
         )}
       </section>
 
+      {/* Concurrency settings */}
+      <section>
+        <div className="flex items-center gap-2 mb-3">
+          <Settings2 size={16} className="text-accent" />
+          <h2 className="text-base font-semibold text-content">Concurrency Settings</h2>
+        </div>
+        <p className="text-xs text-content-muted mb-4">
+          Configure min/max concurrency per queue. The beat scheduler applies these settings
+          every 5 minutes via Celery pool signals. Changes are persisted in the database.
+        </p>
+        {configsLoading ? (
+          <div className="space-y-2">
+            {[0, 1, 2].map((i) => (
+              <div key={i} className="h-16 rounded-xl bg-surface-muted animate-pulse" />
+            ))}
+          </div>
+        ) : !workerConfigs || workerConfigs.length === 0 ? (
+          <p className="text-sm text-content-muted">No worker configs available.</p>
+        ) : (
+          <div className="space-y-2">
+            {workerConfigs.map((cfg) => (
+              <ConcurrencyConfigRow key={cfg.queue_name} config={cfg} />
+            ))}
+          </div>
+        )}
+      </section>
+
       {/* Scale controls */}
       <section>
         <h2 className="text-base font-semibold text-content mb-3">Scale Services</h2>