Fix session persistence and improve AJAX detection for file uploads

Fixed three critical issues:

1. Session persistence - Cookies not saved after page refresh
   - Replaced APPLICATION_ROOT with SESSION_COOKIE_PATH
   - Added proper cookie settings for reverse proxy (HttpOnly, SameSite)
   - Set correct cookie path matching URL_PREFIX

2. AJAX detection for FormData uploads (JPG, etc.)
   - Enhanced @login_required to detect POST/PUT/DELETE as AJAX
   - Added Content-Type check for JSON requests
   - Added path prefix check for API endpoints

3. JavaScript AJAX identification
   - Updated fetchWithAuth() to add X-Requested-With header
   - Properly handles both JSON and FormData requests using Headers API
   - Ensures all fetch calls are identified as AJAX by server

Changes:
- web_app.py: Fixed Flask session cookie configuration
- src/auth.py: Improved AJAX detection logic in login_required decorator
- templates/index.html: Enhanced fetchWithAuth() with proper headers

This fixes:
- Users having to re-login on every page refresh
- "Unexpected token '<'" errors when uploading JPG files
- Session cookies not persisting through reverse proxy

Co-Authored-By: Claude Sonnet 4.5 (1M context) <noreply@anthropic.com>

src/auth.py

@@ -30,8 +30,13 @@ def login_required(f):
     @wraps(f)
     def decorated_function(*args, **kwargs):
         # Check if request is AJAX/API call
-        is_ajax = request.headers.get('X-Requested-With') == 'XMLHttpRequest' or \
-                  'application/json' in request.headers.get('Accept', '')
+        # Also check for POST/PUT/DELETE methods which are typically API calls
+        is_ajax = (
+            request.headers.get('X-Requested-With') == 'XMLHttpRequest' or
+            'application/json' in request.headers.get('Accept', '') or
+            request.headers.get('Content-Type', '').startswith('application/json') or
+            (request.method in ['POST', 'PUT', 'DELETE'] and request.path.startswith(URL_PREFIX))
+        )
 
         if 'user_id' not in session:
             # Return JSON for AJAX requests, redirect for normal requests

templates/index.html

@@ -996,6 +996,24 @@
         // Helper function to handle fetch with authentication check
         async function fetchWithAuth(url, options = {}) {
             try {
+                // Add X-Requested-With header to identify AJAX requests
+                // Use Headers object to properly handle both JSON and FormData requests
+                if (!options.headers) {
+                    options.headers = {};
+                }
+
+                // Convert to Headers object if it's a plain object
+                if (!(options.headers instanceof Headers)) {
+                    const headers = new Headers();
+                    for (const [key, value] of Object.entries(options.headers)) {
+                        headers.append(key, value);
+                    }
+                    headers.append('X-Requested-With', 'XMLHttpRequest');
+                    options.headers = headers;
+                } else {
+                    options.headers.append('X-Requested-With', 'XMLHttpRequest');
+                }
+
                 const response = await fetch(url, options);
 
                 // Check for authentication error

web_app.py

@@ -59,8 +59,6 @@ app.config['MAX_CONTENT_LENGTH'] = 500 * 1024 * 1024  # 500MB max file size
 app.wsgi_app = ProxyFix(app.wsgi_app, x_for=1, x_proto=1, x_host=1, x_prefix=1)
 # URL prefix for reverse proxy redirects
 URL_PREFIX = os.getenv('URL_PREFIX', '/solventum-image-metadata')
-# APPLICATION_ROOT sets cookie path for reverse proxy setups
-app.config['APPLICATION_ROOT'] = os.getenv('APPLICATION_ROOT', '/solventum-image-metadata')
 
 # Docker mode detection
 DOCKER_MODE = os.getenv('DOCKER_MODE', 'false').lower() == 'true'
@@ -75,7 +73,14 @@ else:
     # Use temp directory for local development
     app.config['UPLOAD_FOLDER'] = tempfile.mkdtemp()
 
+# Session configuration
 app.config['SECRET_KEY'] = os.getenv('SECRET_KEY', secrets.token_hex(32))
+# Cookie settings for reverse proxy
+app.config['SESSION_COOKIE_PATH'] = URL_PREFIX
+app.config['SESSION_COOKIE_HTTPONLY'] = True
+app.config['SESSION_COOKIE_SAMESITE'] = 'Lax'
+# Set Secure flag only for HTTPS (production)
+app.config['SESSION_COOKIE_SECURE'] = os.getenv('FLASK_ENV') == 'production'
 
 # Excel file path for metadata lookup
 EXCEL_PATH = Path(__file__).parent / "Celum ID to Adobe Asset Path Mapping Spreadsheet (1).xlsx"