Adobe FDF trust setup — adds the FreeSign CA to local Adobe Reader/Acrobat trust

Returns an Adobe Forms Data Format (FDF) file that, when opened in Adobe Reader or Acrobat, prompts the user to import the FreeSign CA certificate into their local trusted-roots list with usages 'sign documents' and 'certify documents'. This is NOT AATL inclusion and NOT QES — it is a per-user, per-device trust grant. After one-time import, all FreeSign-signed PDFs verify with a green checkmark in that user's Adobe install. The FDF embeds the CA DER and the SHA-256 fingerprint; the response also carries the fingerprint in the x-freesign-ca-sha256 response header for paranoid pre-import checks. Public, GET-only, no authentication; cached for 1 hour.