feat: add VKS-compatible REST endpoints for keys.openpgp.org interface

[bmarwell]

Motivation

To be compatible with the Verifying Keyserver (VKS) API used by keys.openpgp.org (Hagrid), the server should expose:

Method	Path	Description
GET	/vks/v1/by-fingerprint/{fingerprint}	Fetch by full fingerprint
GET	/vks/v1/by-keyid/{keyid}	Fetch by 16-char key ID
GET	/vks/v1/by-email/{email}	Fetch by email (verified UIDs only)
POST	/vks/v1/upload	Upload a key (JSON body: {"keytext": "..."\})
POST	/vks/v1/requestverify	Re-request verification for pending UIDs

All GET responses return application/pgp-keys. All error responses return application/json (or application/problem+json per #133).

Approach

These endpoints can largely delegate to the same ports as the HKP endpoints — KeyRepositoryService for reads and CommandService for writes. A new web/vks war module (or sub-path in web/rest) is appropriate.

Interface note

Defining a Java interface (VksKeyserver) that both the implementation and future test stubs implement would allow integration tests to verify protocol compliance without a running server.