feat: task trash, triage trash and retention settings for every area; admin can forbid keeping things for good

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/api/handlers_admin.go

@@ -340,6 +340,11 @@ func (s *Server) settingsForBrowser(settings *domain.SystemSettings) *domain.Sys
 		out.RegistrationVerifyEmail = s.cfg.RegistrationVerifyEmail == "true"
 		out.RegistrationVerifyViaEnv = true
 	}
+	if s.cfg.RetentionAllowNever != nil {
+		allow := *s.cfg.RetentionAllowNever
+		out.RetentionAllowNever = &allow
+		out.RetentionNeverViaEnv = true
+	}
 	out.EmailLinksAvailable = s.emailLinksAvailable()
 	return &out
 }
@@ -372,6 +377,10 @@ func (s *Server) handleAdminUpdateSettings(w http.ResponseWriter, r *http.Reques
 			settings.RegistrationVerifyEmail = cur.RegistrationVerifyEmail
 		}
 	}
+	if s.cfg.RetentionAllowNever != nil {
+		settings.RetentionAllowNever = nil // fixed by RETENTION_ALLOW_NEVER, the stored value stays
+	}
+	settings.RetentionNeverViaEnv = false
 	previousMode := s.store.SharingMode()
 	if err := s.store.UpdateSystemSettings(&settings); err != nil {
 		writeError(w, http.StatusInternalServerError, err.Error())
@@ -384,7 +393,11 @@ func (s *Server) handleAdminUpdateSettings(w http.ResponseWriter, r *http.Reques
 			return
 		}
 	}
-	writeJSON(w, http.StatusOK, s.settingsForBrowser(&settings))
+	saved, _ := s.store.GetSystemSettings()
+	if saved == nil {
+		saved = &settings
+	}
+	writeJSON(w, http.StatusOK, s.settingsForBrowser(saved))
 }
 
 func (s *Server) handleAdminTestSMTP(w http.ResponseWriter, r *http.Request) {