Fix 502 errors on container restart and expand API authentication

- Handle 502 Gateway Timeout errors by clearing invalid sessions
- Add graceful session validation failure handling in fetch-thread API
- Expand x-tag authentication to accept additional valid token
- Add debug logging to berth-related API endpoints
- Document the 502 error fix in email system documentation

components/EmailThreadView.vue

@@ -215,6 +215,17 @@ const loadEmails = async () => {
     }
   } catch (error: any) {
     console.error('Failed to load emails:', error);
+    
+    // Handle 502 Gateway Timeout - session is likely invalid after container restart
+    if (error.statusCode === 502 || error.status === 502) {
+      console.log('Got 502 error, clearing invalid session');
+      // Clear the invalid session
+      sessionStorage.removeItem('emailSessionId');
+      // Don't show error toast, just reset to no emails
+      threads.value = [];
+      return;
+    }
+    
     if (error.data?.statusMessage?.includes('Email credentials not found')) {
       // Don't show error, parent component should handle reconnection
       threads.value = [];