Sanitize OIDC discovery errors and require CSRF on HEAD and OPTIONS

F03-01: go-oidc embeds the provider's response body in its error for a
non-200 discovery response, which cmd/server logged at startup. Discovery
now runs through a status-recording transport and a non-200 response is
reported as "provider answered HTTP <status>" only; transport errors keep
their cause. A test with a sentinel body proves it reaches neither the
error nor the log.

F03-02: the CSRF middleware exempts only GET, so HEAD and OPTIONS under
/api/v1 without X-MealQuest-CSRF get 403 csrf, matching CONTEXT.MD.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/server/csrf.go

@@ -12,7 +12,7 @@ import (
 // GET /api/v1/me.
 const CSRFHeader = "X-MealQuest-CSRF"
 
-// requireCSRF passes GET, HEAD and OPTIONS requests to next; every other
+// requireCSRF passes GET requests to next; every other
 // request must carry CSRFHeader equal to its session's CSRF token or get 403
 // csrf. It runs behind requireSession, which puts the session in the context.
 func requireCSRF(logger *slog.Logger) func(http.Handler) http.Handler {
@@ -21,7 +21,7 @@ func requireCSRF(logger *slog.Logger) func(http.Handler) http.Handler {
 	return func(next http.Handler) http.Handler {
 		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 			switch r.Method {
-			case http.MethodGet, http.MethodHead, http.MethodOptions:
+			case http.MethodGet:
 				next.ServeHTTP(w, r)
 				return
 			}

internal/server/csrf_test.go

@@ -38,10 +38,9 @@ func TestCSRFThroughRouter(t *testing.T) {
 		{"POST with right header", http.MethodPost, "/api/v1/me", []string{testSession.CSRFToken}, http.StatusMethodNotAllowed, methodNotAllowedJSON},
 		{"PATCH with right header", http.MethodPatch, "/api/v1/settings/household", []string{testSession.CSRFToken}, http.StatusNotFound, notFoundJSON},
 		{"GET without header", http.MethodGet, "/api/v1/me", nil, http.StatusOK, ""},
-		// HEAD and OPTIONS pass the check and reach the router, which
-		// answers neither for /api/v1/me.
-		{"HEAD without header", http.MethodHead, "/api/v1/me", nil, http.StatusMethodNotAllowed, ""},
-		{"OPTIONS without header", http.MethodOptions, "/api/v1/me", nil, http.StatusMethodNotAllowed, methodNotAllowedJSON},
+		// Only GET is exempt; HEAD and OPTIONS need the header too.
+		{"HEAD without header", http.MethodHead, "/api/v1/me", nil, http.StatusForbidden, csrfJSON},
+		{"OPTIONS without header", http.MethodOptions, "/api/v1/me", nil, http.StatusForbidden, csrfJSON},
 	} {
 		t.Run(tc.name, func(t *testing.T) {
 			t.Parallel()
@@ -89,6 +88,8 @@ func TestRequireCSRF(t *testing.T) {
 		{"right header reaches handler", http.MethodPost, &testSession, testSession.CSRFToken, http.StatusNoContent},
 		{"wrong header", http.MethodPut, &testSession, "nope", http.StatusForbidden},
 		{"GET needs no header", http.MethodGet, &testSession, "", http.StatusNoContent},
+		{"HEAD needs the header", http.MethodHead, &testSession, "", http.StatusForbidden},
+		{"OPTIONS needs the header", http.MethodOptions, &testSession, "", http.StatusForbidden},
 		{"no session in context", http.MethodPost, nil, "", http.StatusInternalServerError},
 		{"session without token", http.MethodPost, &auth.Session{Subject: "user-123"}, "", http.StatusInternalServerError},
 	} {

internal/server/server.go

@@ -27,8 +27,7 @@ type RouteMounter interface {
 // only from a peer in trusted (see resolveOrigin). version is reported by
 // GET /health; auth mounts the /auth routes; sessions guards /api/v1 and
 // /media; assets is the web build served for every other GET. /health, /auth
-// and the web build are public. Requests to /api/v1 other than GET, HEAD and
-// OPTIONS also need the session's CSRF token in CSRFHeader.
+// and the web build are public. Requests to /api/v1 other than GET also need the session's CSRF token in CSRFHeader.
 func NewRouter(logger *slog.Logger, version string, trusted forwarded.Trusted, auth RouteMounter, sessions SessionLookup, assets fs.FS) (http.Handler, error) {
 	notFound := apiErrorHandler(logger, http.StatusNotFound, "not_found", "not found")
 	methodNotAllowed := apiErrorHandler(logger, http.StatusMethodNotAllowed, "method_not_allowed", "method not allowed")