feat: unify alerts into one plugin, channels, rules and inbox

plugins/automations/migrations/mysql/0002_unlink_channels.sql

@@ -0,0 +1,15 @@
+-- automations 0002: unlink_channels
+-- Hand-written: channels moved from core's notification_channels to the
+-- alerts plugin, so the link table drops its foreign key to the old table.
+-- The constraint's name differs between installs, so the table is rebuilt.
+
+CREATE TABLE IF NOT EXISTS `p_automations_channels_v2` (
+  `id` int AUTO_INCREMENT PRIMARY KEY,
+  `automation_id` int NOT NULL,
+  `channel_id` int NOT NULL,
+  FOREIGN KEY (`automation_id`) REFERENCES `p_automations_automations` (`id`) ON DELETE CASCADE
+);
+INSERT INTO `p_automations_channels_v2` (`id`, `automation_id`, `channel_id`) SELECT `id`, `automation_id`, `channel_id` FROM `p_automations_channels`;
+DROP TABLE `p_automations_channels`;
+RENAME TABLE `p_automations_channels_v2` TO `p_automations_channels`;
+CREATE UNIQUE INDEX `idx_automation_channels_pair` ON `p_automations_channels` (`automation_id`, `channel_id`);