feat(13-email-mailboxes): phase 13 complete — email DB foundation + SMTP mailbox CRUD

3 migrations (email_mailboxes, email_templates, email_logs), full CRUD
for SMTP mailboxes with encrypted passwords (IntegrationSecretCipher),
native SMTP connection test via stream_socket_client, sidebar navigation.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

database/migrations/20260315_000055_create_email_templates_table.sql

@@ -0,0 +1,13 @@
+CREATE TABLE IF NOT EXISTS `email_templates` (
+    `id` INT UNSIGNED NOT NULL AUTO_INCREMENT,
+    `name` VARCHAR(200) NOT NULL,
+    `subject` VARCHAR(500) NOT NULL,
+    `body_html` TEXT NOT NULL,
+    `mailbox_id` INT UNSIGNED DEFAULT NULL,
+    `is_active` TINYINT(1) NOT NULL DEFAULT 1,
+    `created_at` DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    `updated_at` DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
+    PRIMARY KEY (`id`),
+    KEY `idx_email_templates_mailbox` (`mailbox_id`),
+    CONSTRAINT `fk_email_templates_mailbox` FOREIGN KEY (`mailbox_id`) REFERENCES `email_mailboxes` (`id`) ON DELETE SET NULL
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;