Better validation message for unsupported video dimensions

[huzjie]

Goal

When a request uses unsupported resolution (e.g. 123x456), the API returns a generic 400. Return a clear message listing supported sizes.

Good first issue

• Find the validation in videoforge/api/.
• Add a test asserting the message content.

Acceptance criteria

• 400 response body explains supported dimensions.

[slegarraga]

I'll open a focused PR returning a 400 message that lists the supported resolutions.