feat(push): add service-owned firebase channels and delivery

Add push contracts, provider delivery and channel diagnostics. Link email channels to services and document migration and validation.

Ref: IT-1074

README.md

@@ -4,6 +4,7 @@ Production image references now use `registry.hrynco.com`, preserving the existi
 
 ## Documentation
 
+- [Push channels and Android FCM](docs/android-push.md)
 - [Email template definitions and service languages](docs/email-template-localizations.md)
 - [Service and whole-template API for AI clients](docs/email-template-definition-api.md)
 - [ItemTracker outbox email consumer](docs/itemtracker-outbox-email-consumer.md)
@@ -11,6 +12,9 @@ Production image references now use `registry.hrynco.com`, preserving the existi
 
 ## Windows host development
 
+Incoming RabbitMQ contract names and typed validation are documented in
+[Supported message types](docs/supported-message-types.md).
+
 For the owner's host development workflow, run Web in Rider with the Development
 profile (`localhost:5433` PostgreSQL, HTTP `localhost:5188`) and keep Docker Web
 stopped. Infrastructure can remain in the existing Docker stack. Web does not apply