Add CSRF protection and GET /api/v1/me

RequireSession now stores the validated session in the request context
(auth.WithSession / auth.SessionFrom); auth.ActorFrom derives the actor
from it, so there is one context value.

Every /api/v1 request other than GET, HEAD and OPTIONS must carry
X-MealQuest-CSRF equal to the session's csrf_token (constant-time
compare), else 403 {"error":{"code":"csrf"}}. The check runs after
RequireSession, so unauthenticated requests still get 401. A session
with no token is a 500, never a match against a missing header.

GET /api/v1/me returns {name, email, household: {label: "Our kitchen"},
csrfToken} with Cache-Control: no-store.

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

internal/server/session.go

@@ -15,8 +15,8 @@ type SessionLookup interface {
 }
 
 // requireSession is the RequireSession middleware. It passes requests with a
-// live session to next with the session's actor in the context (read it with
-// auth.ActorFrom); others get 401 unauthenticated.
+// live session to next with the session in the context (read it with
+// auth.SessionFrom or auth.ActorFrom); others get 401 unauthenticated.
 func requireSession(logger *slog.Logger, sessions SessionLookup) func(http.Handler) http.Handler {
 	unauthenticated := apiErrorHandler(logger, http.StatusUnauthorized, "unauthenticated", "sign in required")
 	internal := apiErrorHandler(logger, http.StatusInternalServerError, "internal", "internal error")
@@ -30,7 +30,7 @@ func requireSession(logger *slog.Logger, sessions SessionLookup) func(http.Handl
 				logger.ErrorContext(r.Context(), "look up session", "error", err)
 				internal(w, r)
 			default:
-				next.ServeHTTP(w, r.WithContext(auth.WithActor(r.Context(), s.Actor())))
+				next.ServeHTTP(w, r.WithContext(auth.WithSession(r.Context(), s)))
 			}
 		})
 	}