🔧 Fix authentication and document upload issues

## What was done:
✅ Fixed Firebase Admin initialization to use default credentials for Firebase Functions
✅ Updated frontend to use correct Firebase Functions URL (was using Cloud Run URL)
✅ Added comprehensive debugging to authentication middleware
✅ Added debugging to file upload middleware and CORS handling
✅ Added debug buttons to frontend for troubleshooting authentication
✅ Enhanced error handling and logging throughout the stack

## Current issues:
❌ Document upload still returns 400 Bad Request despite authentication working
❌ GET requests work fine (200 OK) but POST upload requests fail
❌ Frontend authentication is working correctly (valid JWT tokens)
❌ Backend authentication middleware is working (rejects invalid tokens)
❌ CORS is configured correctly and allowing requests

## Root cause analysis:
- Authentication is NOT the issue (tokens are valid, GET requests work)
- The problem appears to be in the file upload handling or multer configuration
- Request reaches the server but fails during upload processing
- Need to identify exactly where in the upload pipeline the failure occurs

## TODO next steps:
1. 🔍 Check Firebase Functions logs after next upload attempt to see debugging output
2. 🔍 Verify if request reaches upload middleware (look for '�� Upload middleware called' logs)
3. 🔍 Check if file validation is triggered (look for '🔍 File filter called' logs)
4. 🔍 Identify specific error in upload pipeline (multer, file processing, etc.)
5. 🔍 Test with smaller file or different file type to isolate issue
6. 🔍 Check if issue is with Firebase Functions file size limits or timeout
7. 🔍 Verify multer configuration and file handling in Firebase Functions environment

## Technical details:
- Frontend: https://cim-summarizer.web.app
- Backend: https://us-central1-cim-summarizer.cloudfunctions.net/api
- Authentication: Firebase Auth with JWT tokens (working correctly)
- File upload: Multer with memory storage for immediate GCS upload
- Debug buttons available in production frontend for troubleshooting

frontend/.env.production

@@ -1,4 +1,4 @@
-VITE_API_BASE_URL=https://api-y56ccs6wva-uc.a.run.app
+VITE_API_BASE_URL=https://us-central1-cim-summarizer.cloudfunctions.net/api
 VITE_FIREBASE_API_KEY=AIzaSyBoV04YHkbCSUIU6sXki57um4xNsvLV_jY
 VITE_FIREBASE_AUTH_DOMAIN=cim-summarizer.firebaseapp.com
 VITE_FIREBASE_PROJECT_ID=cim-summarizer