fix(geocoding): resolve Nominatim 429 rate limit errors

- Add RateLimitFailure to failures.dart for explicit 429 handling
- Increase map picker reverse-geocode debounce from 400ms to 1100ms
  to respect Nominatim's 1 request/second policy

Author: Fury3K

lib/src/core/errors/failures.dart

@@ -41,3 +41,10 @@ class LocationPermissionDeniedForeverFailure extends Failure {
         'Location permission permanently denied. Please enable it in app settings.',
   ]);
 }
+
+class RateLimitFailure extends Failure {
+  const RateLimitFailure([
+    super.message =
+        'Too many requests. Please wait a moment before trying again.',
+  ]);
+}

lib/src/presentation/screens/map_picker_screen.dart

@@ -52,8 +52,9 @@ class _MapPickerScreenState extends State<MapPickerScreen>
   /// during rapid map movements
   Timer? _geocodeDebounceTimer;
 
-  /// Debounce duration for reverse geocoding (400ms)
-  static const Duration _geocodeDebounceDuration = Duration(milliseconds: 400);
+  /// Debounce duration for reverse geocoding.
+  /// Kept above 1100ms to respect Nominatim's 1 request/second policy.
+  static const Duration _geocodeDebounceDuration = Duration(milliseconds: 1100);
 
   // Animation controller for pin bounce effect
   late final AnimationController _pinAnimationController;