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/api/src/dkv/dkv.controller.ts

@@ -55,12 +55,16 @@ export class DkvController {
 
   // ─── Config ────────────────────────────────────────────────────────────────
 
-  /** GET /dkv/config — returns module config without encrypted credentials. */
+  /** GET /dkv/config — returns module config without encrypted credentials. 404 when not yet configured. */
   @Get('config')
   @Roles(Role.ADMIN, Role.SUPER_ADMIN)
   async getConfig(@Req() req: any) {
     const tenantId = this._requireTenant(req);
-    return this.dkvService.loadConfig(tenantId);
+    const config = await this.dkvService.loadConfig(tenantId);
+    if (!config) {
+      throw new NotFoundException('DKV module not yet configured');
+    }
+    return config;
   }
 
   /**