add migration to transform old avatar paths

app/migrations.py

@@ -1,8 +1,13 @@
 from .db import db
 
+def migrate_old_avatars():
+    for avatar in db.query('SELECT id, file_path FROM avatars WHERE file_path LIKE "/avatars/%"'):
+        new_path = f"/static{avatar['file_path']}"
+        db.execute('UPDATE avatars SET file_path = ? WHERE id = ?', new_path, avatar['id'])
+
 # format: [str|tuple(str, any...)|callable]
 MIGRATIONS = [
-
+    migrate_old_avatars,
 ]
 
 def run_migrations():