feat: add FrameRelay Discord launch intents

[vitorhugo-dotnet]

Summary

• Add persistent, expiring share intents and opaque watch capabilities for FrameRelay.
• Restrict bot-only endpoints to a dedicated fixed-time-checked service credential.
• Consume share intents only through the authenticated publisher device, associate them with its normal screen-share session, and resolve viewer capabilities through the existing session join authorization.
• Add a public deep-link landing page, database migration, deployment configuration, and integration tests.

Validation

• API build succeeded.
• The five launch-intent integration tests pass.
• Fixed the receiver-stats WebSocket integration test: bounded the handshake and removed an extra Publisher read for a capability message that the server does not send.
• Removed a duplicate SendAsync helper reintroduced when syncing the branch with main.
• Full integration suite passes locally (244/244).
• GitHub Actions Build and Test jobs pass on commit 6357779 run.

Companion desktop PR: vitorhugo-dotnet/dotnet_FrameRelay#27
Bot implementation PR: vitorhugo-dotnet/go_discord_FrameRelay#1
Implements vitorhugo-dotnet/dotnet_FrameRelay#3