feat: add city avatar upload functionality and enhance city management features

backend/migrations/011_city_avatars.sql

@@ -0,0 +1,6 @@
+-- A city may use a raster coat of arms or another custom image instead of the
+-- text/emoji emblem. The file itself is stored under public/uploads/cities.
+ALTER TABLE cities ADD COLUMN IF NOT EXISTS avatar_url VARCHAR(255);
+
+CREATE INDEX IF NOT EXISTS idx_cities_avatar ON cities(avatar_url)
+WHERE avatar_url IS NOT NULL;