feat(auth): recover cross-signal authorization with provision secrets

A client that could not authorize against a foreign signal server was
redirected into a dead end with no way to retry, so servers joined from
another signal route became unreachable.

The home server now stores a per-user provision secret, clients keep it in
their own store, and a recovery service records why authorization failed per
server URL. Invite, server browser, and chat room surface that reason and
offer a retry instead of silently redirecting.

server/src/migrations/1000000000013-ProvisionSecret.ts

@@ -0,0 +1,13 @@
+import { MigrationInterface, QueryRunner } from 'typeorm';
+
+export class ProvisionSecret1000000000013 implements MigrationInterface {
+  name = 'ProvisionSecret1000000000013';
+
+  async up(queryRunner: QueryRunner): Promise<void> {
+    await queryRunner.query('ALTER TABLE "users" ADD COLUMN "provisionSecret" text');
+  }
+
+  async down(queryRunner: QueryRunner): Promise<void> {
+    await queryRunner.query('ALTER TABLE "users" DROP COLUMN "provisionSecret"');
+  }
+}