Add proof types endpoint

[chetanyb]

Summary

• Add GET /v1/proof_types endpoint that returns configured proof types with their backend
  kind and capabilities (can_prove, can_verify)
• Enables Assertoor and other tooling to discover server capabilities before running tests

Test plan

• Unit tests for handler response and JSON field names
• Unit tests for backend_capabilities() on Ere and Mock variants
• Type serialization tests for BackendKind and ProofTypesResponse
• cargo test --workspace passes
• cargo +nightly fmt --check passes
• cargo clippy passes

Written with Claude Code

[han0110]

LGTM! btw I added the test_verifier_backend_capabilities with the dummy program vk so all are covered.