feat(PROJ-33): IMAP UID-Stabilität + Shared/Personal-Modus

Backend:
- storage: uid BIGSERIAL Migration, MailWithUID, GetMailsWithUID, GetMailsByRecipient
- tenantstore: imap_mode Spalte, GetIMAPMode, SetIMAPMode
- imapserver: stable UIDs aus DB, personal/shared Modus, userEmail in session
- api: GET/PUT /api/admin/settings/imap-mode (domain_admin only, double opt-in)

Frontend:
- IMAPSettingsTab: Modus-Anzeige + Toggle mit Double-Opt-In Dialog
- Admin-Panel: IMAP-Tab für domain_admin

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

internal/tenantstore/store.go

@@ -74,6 +74,7 @@ ALTER TABLE users ADD COLUMN IF NOT EXISTS tenant_id BIGINT REFERENCES tenants(i
 ALTER TABLE audit_log ADD COLUMN IF NOT EXISTS tenant_id BIGINT REFERENCES tenants(id);
 ALTER TABLE tenants ADD COLUMN IF NOT EXISTS logo_data BYTEA;
 ALTER TABLE tenants ADD COLUMN IF NOT EXISTS logo_content_type VARCHAR(100) NOT NULL DEFAULT '';
+ALTER TABLE tenants ADD COLUMN IF NOT EXISTS imap_mode TEXT NOT NULL DEFAULT 'personal';
 `
 
 // New connects to PostgreSQL and initialises the tenant schema.
@@ -326,3 +327,24 @@ func (s *Store) getDomain(ctx context.Context, id int64) (*TenantDomain, error)
 	}
 	return &d, nil
 }
+
+// ── IMAP Mode ─────────────────────────────────────────────────────────────
+
+// GetIMAPMode returns the imap_mode for a tenant ("personal" or "shared").
+func (s *Store) GetIMAPMode(ctx context.Context, tenantID int64) (string, error) {
+	var mode string
+	err := s.pool.QueryRow(ctx, `SELECT imap_mode FROM tenants WHERE id = $1`, tenantID).Scan(&mode)
+	if err != nil {
+		return "personal", nil // safe default
+	}
+	return mode, nil
+}
+
+// SetIMAPMode updates the imap_mode for a tenant. Valid values: "personal", "shared".
+func (s *Store) SetIMAPMode(ctx context.Context, tenantID int64, mode string) error {
+	if mode != "personal" && mode != "shared" {
+		return fmt.Errorf("tenantstore: invalid imap_mode %q", mode)
+	}
+	_, err := s.pool.Exec(ctx, `UPDATE tenants SET imap_mode = $1 WHERE id = $2`, mode, tenantID)
+	return err
+}