feat: API Key 静默认证 + 多邮箱支持

核心改动：
- V3 迁移: accounts 加 api_key/api_secret + account_emails 多邮箱关联表
- HMAC 验签改用 api_secret 替代密码哈希（改密码不影响集成）
- findByAnyEmail 支持主邮箱 + 关联邮箱查找
- AdminController: API Key 生成/重置 + 邮箱绑定/解绑/列表
- 前端 Users.vue: API Key 列 + 生成按钮 + Secret 复制弹窗
- 现有邮箱自动迁移到 account_emails 表

认证流程:
  旧: X-Email → account.email → passwordHash 作 HMAC key
  新: X-Email → account_emails ∪ account.email → api_secret 作 HMAC key

par-core/src/main/resources/db/migration/V3__api_key_multi_email.sql

@@ -0,0 +1,29 @@
+-- PAR (PT Adapter Registry) Schema - Phase 3
+-- Flyway Migration V3: API Key auth + multi-email support
+
+-- accounts 表新增 api_key + api_secret
+ALTER TABLE accounts
+    ADD api_key VARCHAR(64) NULL UNIQUE COMMENT 'API 密钥（public）',
+    ADD api_secret VARCHAR(128) NULL COMMENT 'API 密钥（secret，用于 HMAC 签名）';
+
+-- 为已有管理员账号生成默认 key
+UPDATE accounts SET
+    api_key = CONCAT('par_', LOWER(LEFT(MD5(RAND()), 16))),
+    api_secret = LEFT(SHA2(CONCAT(email, RAND()), 256), 64)
+WHERE api_key IS NULL AND trust_level = 2;
+
+-- 多邮箱关联表
+CREATE TABLE account_emails (
+    id          BIGINT UNSIGNED NOT NULL AUTO_INCREMENT COMMENT '主键',
+    account_id  BIGINT UNSIGNED NOT NULL COMMENT '关联账户ID',
+    email       VARCHAR(255)    NOT NULL COMMENT '邮箱地址',
+    verified    TINYINT(1)      NOT NULL DEFAULT 1 COMMENT '是否已验证',
+    created_at  DATETIME        NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT '绑定时间',
+    PRIMARY KEY (id),
+    UNIQUE KEY uk_email (email),
+    KEY idx_account (account_id)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci COMMENT='账户多邮箱关联表';
+
+-- 现有邮箱迁移到关联表
+INSERT INTO account_emails (account_id, email, verified)
+    SELECT id, email, 1 FROM accounts WHERE deleted = 0;