Fix JPEG MIME type in screenshot data URIs

[iibaranov-IG]

Fixes #1298.

Normalize Qt's jpg writer alias to the registered image/jpeg media type when constructing the GetSourceScreenshot data URI. Other accepted formats keep their existing image/<format> value.

The mapping lives in a small dependency-free helper so it can be exercised without launching OBS.

Validation:

• BCL clean-container regression compiles and executes the production helper with -Wall -Wextra -Werror.
• jpg and jpeg both produce image/jpeg.
• png and webp remain image/png and image/webp.
• BCL verifies that GetSourceScreenshot constructs its response through the tested helper.
• Broadcast Control Lab infrastructure: all 79 tests pass.
• git diff --check passes.

BCL case: https://github.com/iibaranov-IG/broadcast-control-lab/tree/main/cases/obs-websocket-1298