Add login.avatar_id FK for selected avatar (#331)

Adds a nullable foreign key on the login table referencing avatars_list,
making "currently selected avatar" a single authoritative value per user.
The legacy avatars.selected flag is kept (and still writable) for
backwards compatibility; the API mirrors both sides on writes and
provides a reconciler to repair drift.

Author: Brutus5000

migrations/V143__login_avatar_fk.sql

@@ -0,0 +1,15 @@
+-- Add a single "currently selected avatar" reference on the login table.
+-- The legacy `avatars.selected` flag is kept (and still user-writable) for
+-- backwards compatibility; the API mirrors both sides on writes and exposes a
+-- reconciler to repair any drift.
+
+ALTER TABLE login
+  ADD COLUMN avatar_id int(11) unsigned DEFAULT NULL,
+  ADD CONSTRAINT fk_login_avatar
+    FOREIGN KEY (avatar_id) REFERENCES avatars_list (id)
+    ON DELETE SET NULL ON UPDATE CASCADE;
+
+-- Backfill from the legacy selected flag.
+UPDATE login l
+  JOIN avatars a ON a.idUser = l.id AND a.selected = 1
+SET l.avatar_id = a.idAvatar;