Add watch-party exit and close routes; release desktop v0.8.49

[MinionEnjoyer]

What changed

• Adds an authenticated viewer-only watch-party exit route and host-only close route.
• Persists viewer exits so reloads and later sync events do not reopen the player.
• Retains the previous stop route as a compatibility alias.
• Adds targeted realtime dismissal plus REST and gateway contract coverage.
• Fixes JSON null serialization for channels without a visible active party.
• Synchronizes the desktop, web, and API release version at 0.8.49.

Why

Viewers need to dismiss a watch party without ending it for everyone, while hosts need an explicit channel-wide close action. The previous stop-only behavior did not distinguish those responsibilities.

Validation

• API unit suite: 311 tests passed.
• Web unit suite: 74 tests passed.
• Live provider contract: 34 tests passed.
• Chromium interaction suite passed with expected platform skips.
• API and web production builds passed.
• Prisma migration deploy and drift checks passed against temporary PostgreSQL.
• OpenAPI and gateway contracts parse successfully.
• Documentation and secret verification passed.