Migrate storage from JSON to SQLite with enhanced MVC architecture

- Replace JSON file storage with SQLite database for improved concurrency and data integrity
- Implement repository pattern with dedicated model layer (admin_model, file_model, chunk_model)
- Add database.py with automatic migration from existing JSON files
- Enable WAL mode for thread-safe concurrent access across Gunicorn workers
- Store database files in dedicated db/ folder
- Update controllers to use model layer instead of direct JSON access
- Add admin authentication system with TOTP 2FA support

src/models/admin_model.py

@@ -0,0 +1,91 @@
+import sqlite3
+from models.database import get_db
+import utils.mLogger as log
+
+def get_admin():
+    """Get admin user data
+
+    Returns:
+        dict: Admin data with keys: username, password_hash, totp_secret, created_at, last_login
+        None: If no admin exists
+    """
+    try:
+        with get_db() as conn:
+            cursor = conn.cursor()
+            row = cursor.execute("""
+                SELECT username, password_hash, totp_secret, created_at, last_login
+                FROM admin
+                LIMIT 1
+            """).fetchone()
+
+            if row:
+                return dict(row)
+            return None
+    except sqlite3.Error as e:
+        log.e(f"Error getting admin data: {e}")
+        return None
+
+def create_admin(username, password_hash, totp_secret):
+    """Create admin user
+
+    Args:
+        username: Admin username
+        password_hash: Hashed password
+        totp_secret: TOTP secret for 2FA
+
+    Returns:
+        bool: True if successful, False otherwise
+    """
+    try:
+        with get_db() as conn:
+            from datetime import datetime
+
+            conn.execute("""
+                INSERT INTO admin (username, password_hash, totp_secret, created_at)
+                VALUES (?, ?, ?, ?)
+            """, (username, password_hash, totp_secret, datetime.now().isoformat()))
+
+            log.i(f"Admin user created: {username}")
+            return True
+    except sqlite3.Error as e:
+        log.e(f"Error creating admin: {e}")
+        return False
+
+def admin_exists():
+    """Check if an admin user has been created
+
+    Returns:
+        bool: True if admin exists, False otherwise
+    """
+    try:
+        with get_db() as conn:
+            cursor = conn.cursor()
+            count = cursor.execute("SELECT COUNT(*) FROM admin").fetchone()[0]
+            return count > 0
+    except sqlite3.Error as e:
+        log.e(f"Error checking if admin exists: {e}")
+        return False
+
+def update_last_login(username):
+    """Update admin's last login time
+
+    Args:
+        username: Admin username
+
+    Returns:
+        bool: True if successful, False otherwise
+    """
+    try:
+        with get_db() as conn:
+            from datetime import datetime
+
+            conn.execute("""
+                UPDATE admin
+                SET last_login = ?
+                WHERE username = ?
+            """, (datetime.now().isoformat(), username))
+
+            return True
+    except sqlite3.Error as e:
+        log.e(f"Error updating last login: {e}")
+        return False