feat(PROJ-18): E-Mail Integritätsprüfung (SHA-256 Verifikation)

- Storage: VerifyIntegrity, GetAllIDs, GetVerifyStatus + DB-Spalten
- main: Hintergrund-Worker alle 5 Minuten (beim Start sofort: 40/40 OK)
- API: verify_ok + verified_at in GET /api/mails/{id} Antwort
- Frontend: Grüner Haken / graues X / rotes X in Mail-Ansicht

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

src/lib/api.ts

@@ -112,6 +112,8 @@ export interface MailDetail {
   body_plain?: string;
   raw_headers: string;
   attachments: MailAttachment[];
+  verify_ok: boolean | null;
+  verified_at: string | null;
 }
 
 export interface AuditEntry {