Register MiniMax H3 text-to-video configuration

[octo-patch]

Reason: MiniMax H3 text-to-video generation is missing from the registry and its required generation parameters are unavailable in the settings form.

Register MiniMax H3 as the default text-to-video entry with a dedicated credential form using the v2 API endpoint. Expose required 2K resolution and an integer duration control spanning 4–15 seconds, defaulting to 4 seconds, for the existing v2 payload builder.

Validation:

• Python compilation of both changed files passed.
• Ruff 0.15.12 checks (--select E9,F63,F7,F82) passed on both changed files.
• git diff --check and the diff secret scan passed.

Full application tests and live API generation were not run.

[wxg0103]

Thanks — I’ve applied the suggested changes.