fix(dkv): fix inbox processing pipeline — orphan tenant, MIME detection, UNSEEN filter

- Fix orphaned DkvModuleConfig: tenantId pointed to deleted tenant, updated to Default tenant
- DKV controller: return 404 instead of HTTP 200 null when no config exists
- IMAP provider: also detect PDFs sent as application/octet-stream (check filename extension)
- IMAP provider: add seen:false filter so already-processed emails are skipped on re-poll
- IMAP provider: mark email as \Seen after successful PDF download to prevent reprocessing
- Frontend dkv-api: handle 404 from fetchConfig as "not yet configured" (returns null)
- InboxConfigForm: show warning banner when config not yet saved in DB
- InboxConfigForm: add "Jetzt prüfen" button to manually trigger POST /dkv/check-now

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

apps/web/src/lib/dkv-api.ts

@@ -95,10 +95,11 @@ export interface DkvHistoryPage {
  * Fetch the current DKV module inbox configuration.
  * GET /dkv/config
  */
-export async function fetchConfig(): Promise<DkvConfig> {
+export async function fetchConfig(): Promise<DkvConfig | null> {
   const res = await fetch(`${API_URL}/dkv/config`, {
     credentials: 'include',
   });
+  if (res.status === 404) return null;
   if (!res.ok) throw new Error('Failed to fetch DKV config');
   return res.json();
 }
@@ -143,12 +144,13 @@ export async function testConnection(
  * Trigger an immediate inbox check (manual poll).
  * POST /dkv/check-now
  */
-export async function checkNow(): Promise<void> {
+export async function checkNow(): Promise<{ status: string; message: string; checkedAt: string }> {
   const res = await fetch(`${API_URL}/dkv/check-now`, {
     method: 'POST',
     credentials: 'include',
   });
   if (!res.ok) throw new Error('Failed to trigger DKV inbox check');
+  return res.json();
 }
 
 /**