975 File Upload Executor Fix 1c8c7809

# File Upload Executor Fix

**Version Implemented:** 0.229.001

## Issue Description
File uploads were failing with a 400 Bad Request error in versions 0.228.004+ while working in version 0.227.015. The browser console showed:
```
POST https://127.0.0.1:5000/api/documents/upload 400 (BAD REQUEST)
```

## Root Cause Analysis
The issue was caused by refactoring changes between versions 0.227.015 and 0.228.004+ where:

1. **Flask app setup moved from `config.py` to `app.py`**
2. **Executor instance creation moved to `app.py`**
3. **Route files no longer had access to the `executor` variable**

The specific error was:
```json
{
  "errors": [
    "Failed to queue processing for test.txt: name 'executor' is not defined"
  ]
}
```

## Technical Details

### Files Modified
- `route_backend_documents.py`: Fixed executor access in upload endpoints
- `config.py`: Added missing SECRET_KEY configuration constant  
- `app.py`: Added SECRET_KEY to Flask app configuration

### Code Changes Summary

#### 1. Executor Access Fix
**Before:**
```python
future = executor.submit_stored(
    parent_document_id, 
    process_document_upload_background,
    # ... parameters
)
```

**After:**
```python
from flask import current_app

future = current_app.extensions['executor'].submit_stored(
    parent_document_id, 
    process_document_upload_background,
    # ... parameters  
)
```

#### 2. Session Configuration Fix
**Added to config.py:**
```python
SECRET_KEY = os.getenv('SECRET_KEY', 'dev-secret-key-change-in-production')
```

**Added to app.py:**
```python
app.config['SECRET_KEY'] = SECRET_KEY
```

### Testing Approach
- Created functional test to validate session configuration
- Created diagnostic test to identify validation failure points
- Tested upload endpoint with simulated authenticated session
- Verified temp file creation and blob storage integration

## Validation

### Test Results
- ✅ **Session persistence working** - SECRET_KEY properly configured
- ✅ **User workspace enabled** - Admin setting verified
- ✅ **File type validation working** - Allowed extensions validated
- ✅ **Upload endpoint responding** - Status 200 with successful processing
- ✅ **Background processing working** - Executor properly accessible
- ✅ **Blob storage integration** - Files saved to Azure Blob Storage
- ✅ **Document metadata creation** - Cosmos DB documents created

### Before/After Comparison
**Before Fix:**
- Upload attempts resulted in 400 Bad Request
- Backend error: `name 'executor' is not defined`
- Session authentication failing due to missing SECRET_KEY

**After Fix:**
- Upload attempts return 200 Success
- Files properly queued for background processing
- Document metadata created in Cosmos DB
- Files uploaded to blob storage successfully

## User Experience Improvements
- File uploads now work reliably in workspace interface
- Proper error handling with informative messages
- Background processing for large files works correctly
- Session-based authentication functions properly

## Impact Analysis
This fix restores file upload functionality that was broken by the Flask app configuration refactoring. It affects:
- **Workspace document uploads** - Primary functionality restored
- **Background file processing** - Executor access restored
- **Session management** - Authentication stability improved
- **User workflow** - Upload feature fully functional

## Related Files
- `functional_tests/test_upload_diagnosis.py` - Diagnostic test
- `functional_tests/test_session_configuration.py` - Session validation test
- `route_backend_documents.py` - Upload endpoint implementation
- `config.py` - Configuration constants
- `app.py` - Flask application setup