rtmp-services: Add Twitcast (ツイキャス)

[shiba-moi]

Description

Add Twitcast to the streaming services list.

Motivation and Context

Twitcast users currently need to manually enter the RTMP URL as a custom server.
Adding it as a built-in service simplifies setup and allows centralized URL management.

How Has This Been Tested?

• Replaced services.json locally and confirmed the service appears in the service dropdown
• Verified streaming to the ingest endpoint

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.