Swap a push provider

• Interface: PushTransport (send(token, message): Promise<PushResult>) in apps/notifications/src/channels/push/push-transport.ts, one per platform (ios, android, web).
• Today: FcmTransport (fcm.ts, Android), ApnsTransport (apns.ts, iOS), WebPushTransport (web-push.ts, browsers, VAPID).
• Selected in: createTransports() in apps/notifications/src/channels/push/push.module.ts; a platform is on when all of its variables are set (pushPlatforms in apps/notifications/src/env.ts).
• Env: FCM_PROJECT_ID, FCM_CLIENT_EMAIL, FCM_PRIVATE_KEY; APNS_KEY_ID, APNS_TEAM_ID, APNS_PRIVATE_KEY, APNS_BUNDLE_ID, APNS_URL; VAPID_PUBLIC_KEY, VAPID_PRIVATE_KEY, VAPID_SUBJECT (the public key also goes to apps/api, which hands it to browsers). Test hooks: FCM_TOKEN_URL, FCM_API_URL, WEB_PUSH_TEST_ORIGIN.

Swap

1. A class implementing PushTransport. Return { ok: false, gone: true } for dead tokens (uninstalled app, expired subscription) so the device is forgotten, and gone: false for failures worth retrying.
2. Its variables in apps/notifications/src/env.ts (all or none, like the others), .env.example and docs/environment.md.
3. Register it for its platform in createTransports(). The channel, dispatcher and quiet hours don't change. A provider that uses its own device tokens (Expo push) also changes how apps register devices (apps/mobile, the notifications procedures).

Tests

apps/notifications/test/fake-push.ts fakes FCM, APNs and Web Push; extend it or add a fake for the new provider, and cover delivery and a dead token in apps/notifications/test/notifications.integration.test.ts.

Finish

1. The verify skill.
2. Ask the reviewer agent to review the change, and the security-reviewer agent: a new implementation brings its own credentials and sends data somewhere new. If you wrote a migration, the migration-reviewer agent too.
3. Update the seam's row in the README's "Scaling path" table if what's "Now" changed.