feat(rate-limit): per-user mutation-rate-limiting (v1-readiness #3)

lib/rate-limit.ts: 11 nieuwe scope-configs + enforceUserRateLimit(scope, userId)
helper. Returnt { error, code: 429 } shape voor consistent foutbeleid.

Toegepast op de high-value mutation-paths:
- actions/pbis.ts createPbiAction
- actions/stories.ts createStoryAction
- actions/tasks.ts saveTask (alleen create-path) + createTaskAction
- actions/todos.ts createTodoAction
- actions/sprints.ts createSprintAction
- actions/products.ts createProductAction + createProductFormAction
- actions/api-tokens.ts createApiTokenAction
- actions/questions.ts answerQuestion
- actions/claude-jobs.ts enqueueClaudeJobAction + enqueueClaudeJobsBatchAction
- app/api/profile/avatar/route.ts POST
- app/api/stories/[id]/log/route.ts POST

Limits zijn ruim genoeg voor normaal gebruik, eng genoeg voor abuse-loops:
create-task 100/min, create-todo 60/min, create-pbi 30/min, create-product
5/min, create-token 10/uur, etc. Per-user scope (geen globale block).

Niet aangeraakt: reorder/status-toggle (intra-session frequent, lage abuse),
update/delete (laag-volume), cron-routes (CRON_SECRET-gated).

Consumer-tweaks: 'success' in result narrowing waar TS de bredere union niet
meer accepteerde. Tests: 9 nieuwe op rate-limit-helper.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

actions/pbis.ts

@@ -10,6 +10,7 @@ import { isValidCode, normalizeCode } from '@/lib/code'
 import { createWithCodeRetry, generateNextPbiCode } from '@/lib/code-server'
 import { pbiStatusFromApi } from '@/lib/task-status'
 import { createPbiSchema, updatePbiSchema } from '@/lib/schemas/pbi'
+import { enforceUserRateLimit } from '@/lib/rate-limit'
 
 async function getSession() {
   return getIronSession<SessionData>(await cookies(), sessionOptions)
@@ -22,6 +23,9 @@ export async function createPbiAction(_prevState: unknown, formData: FormData) {
   if (!session.userId) return { error: 'Niet ingelogd', code: 403 }
   if (session.isDemo) return { error: 'Niet beschikbaar in demo-modus', code: 403 }
 
+  const limited = enforceUserRateLimit('create-pbi', session.userId)
+  if (limited) return limited
+
   const parsed = createPbiSchema.safeParse({
     productId: formData.get('productId'),
     code: (formData.get('code') as string) || undefined,