fix: make flask-limiter import optional to prevent worker boot failure

flask-limiter may not be installed (e.g. RPi venv). The hard import
crashed the gunicorn gevent worker on startup, causing all routes to
return 404 with no visible error. Now falls back to a no-op limiter.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app.py

@@ -42,8 +42,12 @@ from utils.constants import (
     QUEUE_MAX_SIZE,
 )
 import logging
-from flask_limiter import Limiter
-from flask_limiter.util import get_remote_address
+try:
+    from flask_limiter import Limiter
+    from flask_limiter.util import get_remote_address
+    _has_limiter = True
+except ImportError:
+    _has_limiter = False
 # Track application start time for uptime calculation
 import time as _time
 _app_start_time = _time.time()
@@ -53,12 +57,21 @@ logger = logging.getLogger('intercept.database')
 app = Flask(__name__)
 app.secret_key = "signals_intelligence_secret" # Required for flash messages
 
-# Set up rate limiting
-limiter = Limiter(
-    key_func=get_remote_address, # Identifies the user by their IP
-    app=app,
-    storage_uri="memory://", # Use RAM memory (change to redis:// etc. for distributed setups)
-)
+# Set up rate limiting (optional — flask-limiter may not be installed)
+if _has_limiter:
+    limiter = Limiter(
+        key_func=get_remote_address,
+        app=app,
+        storage_uri="memory://",
+    )
+else:
+    class _NoopLimiter:
+        """Fallback when flask-limiter is not installed."""
+        def limit(self, *args, **kwargs):
+            def decorator(f):
+                return f
+            return decorator
+    limiter = _NoopLimiter()
 
 # Disable Werkzeug debugger PIN (not needed for local development tool)
 os.environ['WERKZEUG_DEBUG_PIN'] = 'off'