feat(core): Set up leader selection for multiple main instances (#7527)

https://linear.app/n8n/issue/PAY-933/set-up-leader-selection-for-multiple-main-instances

- [x] Set up new envs
- [x] Add config and license checks
- [x] Implement `MultiMainInstancePublisher`
- [x] Expand `RedisServicePubSubPublisher` to support
`MultiMainInstancePublisher`
- [x] Init `MultiMainInstancePublisher` on startup and destroy on
shutdown
- [x] Add to sandbox plans
- [x] Test manually

Note: This is only for setup - coordinating in reaction to leadership
changes will come in later PRs.

packages/cli/src/config/schema.ts

@@ -1323,4 +1323,25 @@ export const schema = {
 			env: 'N8N_WORKFLOW_HISTORY_PRUNE_TIME',
 		},
 	},
+
+	leaderSelection: {
+		enabled: {
+			doc: 'Whether to enable leader selection for multiple main instances (license required)',
+			format: Boolean,
+			default: false,
+			env: 'N8N_LEADER_SELECTION_ENABLED',
+		},
+		ttl: {
+			doc: 'Time to live in Redis for leader selection key, in seconds',
+			format: Number,
+			default: 10,
+			env: 'N8N_LEADER_SELECTION_KEY_TTL',
+		},
+		interval: {
+			doc: 'Interval in Redis for leader selection check, in seconds',
+			format: Number,
+			default: 3,
+			env: 'N8N_LEADER_SELECTION_CHECK_INTERVAL',
+		},
+	},
 };