feat: one account can bind both netease + skin identities

- db: user_identities table (UNIQUE user_id+source), migrate backfill from users
- auth: GET/POST/DELETE /api/auth/identities - bind/unbind/list identities
  (netease requires UID, one per source, name uniqueness, keep >=1)
- register/external/plugin/admin-created users auto-write primary identity
- tickets: submit uses selected identity (validated belongs to user)
- dashboard: 我的身份 card with bind/unbind UI
- ticket-create: identity selector when >1 identity

backend/routes/external.js

@@ -29,7 +29,11 @@ router.post('/auth/register', async (req, res) => {
 
     const hashed = bcrypt.hashSync(password, 10);
     const verifyToken = uuid();
-    await query('INSERT INTO users(username,password,email,game_name,game_uid,verify_token,verify_expires,source) VALUES (?,?,?,?,?,?,DATE_ADD(NOW(), INTERVAL 24 HOUR),?)', [username, hashed, email, game_name, game_uid,       verifyToken, source || 'skin']);
+    const r = await query('INSERT INTO users(username,password,email,game_name,game_uid,verify_token,verify_expires,source) VALUES (?,?,?,?,?,?,DATE_ADD(NOW(), INTERVAL 24 HOUR),?)', [username, hashed, email, game_name, game_uid, verifyToken, source || 'skin']);
+    try {
+      await query('INSERT INTO user_identities(user_id, source, game_name, game_uid) VALUES (?,?,?,?)',
+        [r.insertId, source || 'skin', game_name, game_uid]);
+    } catch {}
 
     const site = await getRow("SELECT v FROM settings WHERE k='site_url'");
     const sent = await sendEmail(email, 'verify_email', { username, game_name, game_uid, verify_link: `${site?.v||'http://localhost:3100'}#/verify?token=${verifyToken}` });