Fix superadmin matching: case-insensitive username/email lookup

Azure AD returns mixed-case usernames (VadymSamoilenko) but
superadmin was created with lowercase. Use LOWER() for matching.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/database.py

@@ -157,10 +157,13 @@ class Database:
             logger.error(f"Error creating test user: {e}")
 
     def _create_superadmin(self, conn: sqlite3.Connection, email: str):
-        """Create or promote superadmin user."""
+        """Create or promote superadmin user (case-insensitive match)."""
         try:
             username = email.split('@')[0]
-            cursor = conn.execute('SELECT id, role FROM users WHERE username = ? OR email = ?', (username, email))
+            cursor = conn.execute(
+                'SELECT id, role FROM users WHERE LOWER(username) = LOWER(?) OR LOWER(email) = LOWER(?)',
+                (username, email),
+            )
             row = cursor.fetchone()
             if row:
                 if row['role'] != 'admin':