feat(PROJ-29): Tenant-Quotas & Usage-Limits

- DB: max_storage_bytes, max_users, max_emails per Tenant (NULL = unlimited)
- storage/quota.go: CheckQuota() mit 60s-Cache, ErrQuotaExceeded
- Save() prüft Quota vor dem Schreiben — Ablehnung bei Hard-Limit
- tenantstore/quota.go: SetQuota(), GetQuota(), GetUsage()
- API: GET/PUT /api/admin/tenant/{id}/quota, GET /api/admin/quotas
- QuotaTab: Usage-Balken (Speicher/Nutzer/Mails), Edit-Dialog, Warnung ab 80%
- InvalidateQuotaCache() nach Quota-Änderung für sofortige Wirkung

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

internal/storage/storage.go

@@ -270,7 +270,12 @@ func (s *Store) Save(ctx context.Context, raw []byte, _ time.Time, tenantID *int
 		}
 	}
 
-	// Step 3: SHA-256-based dedup (fallback / no Message-ID)
+	// Step 3: Quota check (PROJ-29) — reject before writing
+	if err := s.CheckQuota(ctx, tenantID); err != nil {
+		return "", err
+	}
+
+	// Step 4: SHA-256-based dedup (fallback / no Message-ID)
 	sum := sha256.Sum256(raw)
 	id := fmt.Sprintf("%x", sum[:]) // 64 hex chars