Refactor application to MVC architecture with Flask blueprints

Major architectural refactoring to improve code organization, maintainability, and scalability:

- Implement MVC pattern with clear separation of concerns
- Organize code into blueprints for different functional areas (main, tenant, admin, api)
- Create models layer to encapsulate all database operations
- Create services layer for business logic (auth, product, barcode)
- Implement application factory pattern for better testability
- Add environment-based configuration management
- Remove old monolithic app.py and consolidate routes

New structure:
- app/models/ - Database operations (Tenant, Product, ARField, Settings)
- app/services/ - Business logic (Auth, Product, Barcode services)
- app/blueprints/ - Controllers organized by function
- app/config.py - Environment configurations
- run.py - Application entry point

All routes remain backward compatible. No changes to API or templates functionality.

src/app/models/settings.py

@@ -0,0 +1,30 @@
+from typing import Optional
+from .base import get_db
+
+class SettingsModel:
+    """Model for application settings"""
+
+    @staticmethod
+    def get(key: str, default: Optional[str] = None) -> Optional[str]:
+        """Get a setting value by key"""
+        with get_db() as conn:
+            cursor = conn.cursor()
+            cursor.execute('SELECT value FROM settings WHERE key = ?', (key,))
+            row = cursor.fetchone()
+            return row['value'] if row else default
+
+    @staticmethod
+    def set(key: str, value: str):
+        """Set a setting value"""
+        with get_db() as conn:
+            cursor = conn.cursor()
+            cursor.execute('''
+                INSERT OR REPLACE INTO settings (key, value, updated_at)
+                VALUES (?, ?, CURRENT_TIMESTAMP)
+            ''', (key, value))
+            conn.commit()
+
+    @staticmethod
+    def get_server_url() -> str:
+        """Get server URL setting"""
+        return SettingsModel.get('server_url', 'http://localhost:5555')