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/db.js

@@ -88,6 +88,17 @@ async function initSchema(connection) {
     updated_at DATETIME DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP
   ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4`);
 
+  await createIfNotExists('user_identities', `CREATE TABLE user_identities (
+    id INT AUTO_INCREMENT PRIMARY KEY,
+    user_id INT NOT NULL,
+    source ENUM('netease','skin') NOT NULL,
+    game_name VARCHAR(50) NOT NULL,
+    game_uid VARCHAR(50) NOT NULL DEFAULT '',
+    created_at DATETIME DEFAULT CURRENT_TIMESTAMP,
+    UNIQUE KEY uk_user_source (user_id, source),
+    INDEX idx_user (user_id)
+  ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4`);
+
   await createIfNotExists('tickets', `CREATE TABLE tickets (
     id INT AUTO_INCREMENT PRIMARY KEY,
     user_id INT,
@@ -291,6 +302,22 @@ async function migrateAdditions(db) {
   try { await db.execute(`CREATE TABLE IF NOT EXISTS bans (id INT AUTO_INCREMENT PRIMARY KEY, player_name VARCHAR(50) NOT NULL, player_uid VARCHAR(50), source VARCHAR(10) DEFAULT '', reason TEXT, type ENUM('ban','mute','warn','other') NOT NULL DEFAULT 'ban', duration VARCHAR(20) DEFAULT '', ticket_id INT, created_by INT, status ENUM('active','expired','appealed','lifted') NOT NULL DEFAULT 'active', created_at DATETIME DEFAULT CURRENT_TIMESTAMP, expires_at DATETIME) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4`); } catch {}
   try { await db.execute("ALTER TABLE bans ADD COLUMN source VARCHAR(10) DEFAULT ''"); } catch {}
 
+  // 多来源身份: 建表 + 从 users 主身份回填(幂等)
+  try {
+    await db.execute(`CREATE TABLE IF NOT EXISTS user_identities (
+      id INT AUTO_INCREMENT PRIMARY KEY,
+      user_id INT NOT NULL,
+      source ENUM('netease','skin') NOT NULL,
+      game_name VARCHAR(50) NOT NULL,
+      game_uid VARCHAR(50) NOT NULL DEFAULT '',
+      created_at DATETIME DEFAULT CURRENT_TIMESTAMP,
+      UNIQUE KEY uk_user_source (user_id, source),
+      INDEX idx_user (user_id)
+    ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4`);
+    await db.execute(`INSERT IGNORE INTO user_identities (user_id, source, game_name, game_uid)
+      SELECT id, source, game_name, game_uid FROM users WHERE game_name != ''`);
+  } catch {}
+
   try { await db.execute(`CREATE TABLE IF NOT EXISTS polls (id INT AUTO_INCREMENT PRIMARY KEY, title VARCHAR(200) NOT NULL, description TEXT, options JSON NOT NULL, group_name VARCHAR(100) NOT NULL DEFAULT '', server_name VARCHAR(100) NOT NULL DEFAULT '', start_time DATETIME, end_time DATETIME, active TINYINT(1) NOT NULL DEFAULT 1, created_by INT, created_at DATETIME DEFAULT CURRENT_TIMESTAMP, INDEX idx_active (active), INDEX idx_group_server (group_name, server_name)) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4`); } catch {}
   try { await db.execute(`CREATE TABLE IF NOT EXISTS poll_votes (id INT AUTO_INCREMENT PRIMARY KEY, poll_id INT NOT NULL, user_id INT NOT NULL, option_index INT NOT NULL, created_at DATETIME DEFAULT CURRENT_TIMESTAMP, UNIQUE KEY uk_poll_user (poll_id, user_id), INDEX idx_poll (poll_id)) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4`); } catch {}
   try { await db.execute(`CREATE TABLE IF NOT EXISTS feature_items (id INT AUTO_INCREMENT PRIMARY KEY, title VARCHAR(200) NOT NULL, description TEXT, group_name VARCHAR(100) NOT NULL DEFAULT '', server_name VARCHAR(100) NOT NULL DEFAULT '', status ENUM('pending','planned','done') NOT NULL DEFAULT 'pending', created_at DATETIME DEFAULT CURRENT_TIMESTAMP) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4`); } catch {}