Sign the agreement mandate

Stamps the signer's signature onto the generated mandate PDF and stores the result as the entry's signed agreement. Satisfies the sign step on the approval flow.

Two modes:

• Drawn: pass a PNG or JPEG image (≤ 4 MB) in signature. The image is inked onto the signature box.
• Typed: omit signature or send an empty value. The signer's name is rendered as a typed signature.

Requires the silo entry to have signer fields set (given name, surname, and identity number) before calling. Use together with POST /identity, then call POST /confirm to finalise.