fix: API Routes — Ergebnisprüfung, Suppression-Check vor Enqueue, Bulk-Enqueue, Status-Codes, Draft-Guard

- getTenantId in shared Utility src/lib/tenant-header.ts ausgelagert (alle 4 Route-Dateien)
- send/route.ts: updateCampaignStatus-Ergebnis geprüft (C1), addBulk statt sequentiellem await (C2), Suppression-Check vor Enqueue (C3)
- [id]/route.ts: GET unterscheidet 404/500, PATCH unterscheidet 404/400
- schedule/route.ts: immediate-Typ abgelehnt (I4), Draft-Guard hinzugefügt (I5)
- .eslintrc.json ergänzt (next/core-web-vitals) — fehlte im Projekt

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

src/app/api/campaigns/route.ts

@@ -1,10 +1,7 @@
 import { NextRequest, NextResponse } from 'next/server'
 import { CreateCampaignSchema } from '../../../lib/validation'
 import { createCampaign, listCampaigns } from '../../../server/db/campaigns'
-
-function getTenantId(req: NextRequest): string {
-  return req.headers.get('x-tenant-id') ?? 'default'
-}
+import { getTenantId } from '../../../lib/tenant-header'
 
 export async function GET(req: NextRequest) {
   const tenantId = getTenantId(req)