feat(relay): OAuth token endpoint — DPoP, PKCE, ES256 JWT, refresh rotation (MM-77) · pull #45

Summary#

Implements POST /oauth/token with authorization_code and refresh_token grants (RFC 6749)
DPoP proof validation with server-issued nonces (RFC 9449) — enforced on all token requests
PKCE S256 verification using constant-time comparison to prevent timing oracles
ES256 access tokens in AT+JWT format with cnf.jkt DPoP binding (RFC 9068)
Single-use refresh token rotation with atomic SELECT+DELETE to prevent TOCTOU races
Persistent P-256 signing keypair (AES-256-GCM encrypted at rest)
V012 migration: jkt column on oauth_tokens, oauth_signing_key table

cargo test -p relay — 334 tests, 0 failures
cargo clippy -p relay -- -D warnings — 0 warnings
Human test plan: docs/test-plans/2026-03-22-MM-77.md (30/30 AC automated; AC6.2 needs manual restart verification)