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

plugins/automations/migrations/sqlite/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.
+-- SQLite cannot drop a constraint, so the table is rebuilt.
+
+CREATE TABLE IF NOT EXISTS "p_automations_channels_v2" (
+  "id" integer PRIMARY KEY AUTOINCREMENT,
+  "automation_id" integer NOT NULL,
+  "channel_id" integer 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";
+ALTER TABLE "p_automations_channels_v2" RENAME TO "p_automations_channels";
+CREATE UNIQUE INDEX IF NOT EXISTS "idx_automation_channels_pair" ON "p_automations_channels" ("automation_id", "channel_id");