feat(PROJ-34): Retention-Tab + pro-Mandant Aufbewahrungsfristen

- tenantstore: retention_days Spalte, GetRetentionDays/SetRetentionDays
- storage.Save(): per-tenant retention überschreibt globale config
- API: GET /api/admin/retention, PUT /api/admin/tenant/{id}/retention
- Frontend: RetentionTab mit globaler Policy-Anzeige, Mandanten-Tabelle,
  Bearbeiten-Dialog und Purge-Button (superadmin only)

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

internal/tenantstore/store.go

@@ -20,11 +20,12 @@ type Tenant struct {
 	Active    bool      `json:"active"`
 	CreatedAt time.Time `json:"created_at"`
 	// Computed fields populated by List.
-	DomainCount int    `json:"domain_count,omitempty"`
-	UserCount   int    `json:"user_count,omitempty"`
-	LDAPEnabled *bool  `json:"ldap_enabled,omitempty"`
-	LDAPURL     string `json:"ldap_url,omitempty"`
-	HasLogo     bool   `json:"has_logo,omitempty"`
+	DomainCount   int    `json:"domain_count,omitempty"`
+	UserCount     int    `json:"user_count,omitempty"`
+	LDAPEnabled   *bool  `json:"ldap_enabled,omitempty"`
+	LDAPURL       string `json:"ldap_url,omitempty"`
+	HasLogo       bool   `json:"has_logo,omitempty"`
+	RetentionDays int    `json:"retention_days"` // 0 = use global config
 }
 
 // TenantDomain is an e-mail domain assigned to a tenant.
@@ -75,6 +76,7 @@ ALTER TABLE audit_log ADD COLUMN IF NOT EXISTS tenant_id BIGINT REFERENCES tenan
 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';
+ALTER TABLE tenants ADD COLUMN IF NOT EXISTS retention_days INT NOT NULL DEFAULT 0;
 `
 
 // New connects to PostgreSQL and initialises the tenant schema.
@@ -125,7 +127,8 @@ func (s *Store) List(ctx context.Context) ([]Tenant, error) {
 		       COUNT(DISTINCT u.id)          AS user_count,
 		       tl.enabled                    AS ldap_enabled,
 		       tl.url                        AS ldap_url,
-		       (t.logo_data IS NOT NULL)     AS has_logo
+		       (t.logo_data IS NOT NULL)     AS has_logo,
+		       t.retention_days
 		FROM tenants t
 		LEFT JOIN tenant_domains td ON td.tenant_id = t.id
 		LEFT JOIN users u           ON u.tenant_id  = t.id
@@ -141,7 +144,7 @@ func (s *Store) List(ctx context.Context) ([]Tenant, error) {
 	var tenants []Tenant
 	for rows.Next() {
 		var t Tenant
-		if err := rows.Scan(&t.ID, &t.Name, &t.Slug, &t.Active, &t.CreatedAt, &t.DomainCount, &t.UserCount, &t.LDAPEnabled, &t.LDAPURL, &t.HasLogo); err != nil {
+		if err := rows.Scan(&t.ID, &t.Name, &t.Slug, &t.Active, &t.CreatedAt, &t.DomainCount, &t.UserCount, &t.LDAPEnabled, &t.LDAPURL, &t.HasLogo, &t.RetentionDays); err != nil {
 			return nil, fmt.Errorf("tenantstore: scan: %w", err)
 		}
 		tenants = append(tenants, t)
@@ -348,3 +351,30 @@ func (s *Store) SetIMAPMode(ctx context.Context, tenantID int64, mode string) er
 	_, err := s.pool.Exec(ctx, `UPDATE tenants SET imap_mode = $1 WHERE id = $2`, mode, tenantID)
 	return err
 }
+
+// ── Retention ─────────────────────────────────────────────────────────────────
+
+// GetRetentionDays returns the per-tenant retention_days (0 = use global config).
+func (s *Store) GetRetentionDays(ctx context.Context, tenantID int64) (int, error) {
+	var days int
+	err := s.pool.QueryRow(ctx, `SELECT retention_days FROM tenants WHERE id = $1`, tenantID).Scan(&days)
+	if err != nil {
+		return 0, nil // safe default: fall back to global
+	}
+	return days, nil
+}
+
+// SetRetentionDays sets the per-tenant retention_days. 0 means "use global config".
+func (s *Store) SetRetentionDays(ctx context.Context, tenantID int64, days int) error {
+	if days < 0 {
+		return fmt.Errorf("tenantstore: retention_days must be >= 0")
+	}
+	tag, err := s.pool.Exec(ctx, `UPDATE tenants SET retention_days = $1 WHERE id = $2`, days, tenantID)
+	if err != nil {
+		return fmt.Errorf("tenantstore: set retention: %w", err)
+	}
+	if tag.RowsAffected() == 0 {
+		return fmt.Errorf("tenantstore: tenant %d not found", tenantID)
+	}
+	return nil
+}