Harden the authorization-code grant and UserInfo verification

Closes the local protocol surface of gap G01 from the scope assessment
(KEY-WP-0016). The browser grant validated PKCE, client id and scopes but left
four bindings unenforced, and UserInfo verified less than the caller CLI does.

Authorization-code path: bind the exchange to the redirect URI the code was
issued for, refuse clients whose registration does not permit the grant, and
authenticate confidential clients with a digest-based constant-time comparison
over the same credential sources as the service grant. An empty grantTypes stays
an implicit authorization-code client, matching config validation.

Code consumption: SessionStore.Consume reads and deletes under one lock. The
previous Get/Delete pair spanned JWT signing, and the added test reproduces the
race against that version -- 9 of 16 concurrent exchanges succeeded, and a
failed exchange left the code replayable.

UserInfo: check the JOSE header algorithm before trusting the signature, require
the configured issuer, and require an access token rather than accepting an ID
token of the right shape. Purpose is decided on the scope claim so the issued
token contract, which consumers pin exactly, does not change.

SCOPE.md and the assessment record which bindings are now enforced and that the
Authelia upstream-trust assumption remains open, so G01 is not fully closed and
no profile-conformance claim is made.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NV9oijZukGyGbRQGGKnK4P

Assistant: claude-code
Assistant-Model: opus
Assistant-Process: 713576@bnt-lap001
Assistant-Session: 384c511d-9bce-4cb8-a676-2aef6c0c8df6

src/internal/server/oidc/session.go

@@ -69,6 +69,28 @@ func (s *SessionStore) Get(code string) (*PKCESession, bool) {
 	return sess, true
 }
 
+// Consume retrieves a session by code and removes it in the same critical
+// section, so exactly one caller can ever observe a given code. Returns false
+// if the code is not present or has expired. The token endpoint must use this
+// rather than Get/Delete: signing happens between those two calls, which is
+// long enough for two concurrent exchanges to both observe the same session
+// (KEY-WP-0016-T01). A consumed code is gone even if the exchange then fails,
+// which is the intended single-use semantics -- a failed attempt must not leave
+// a replayable code.
+func (s *SessionStore) Consume(code string) (*PKCESession, bool) {
+	s.mu.Lock()
+	sess, ok := s.sessions[code]
+	if ok {
+		delete(s.sessions, code)
+	}
+	s.mu.Unlock()
+
+	if !ok || time.Now().After(sess.ExpiresAt) {
+		return nil, false
+	}
+	return sess, true
+}
+
 // Delete removes a session by code. No-op if the code is not present.
 func (s *SessionStore) Delete(code string) {
 	s.mu.Lock()