debug: add logging to AcceptInvite to trace 404

internal/handler/handler.go

···
 func (h *Handler) AcceptInvite(w http.ResponseWriter, r *http.Request) {
 	user := h.currentUser(r)
 	if user == nil {
+		log.Printf("AcceptInvite: unauthenticated, redirecting to login")
 		// Preserve invite token through the login redirect.
 		http.Redirect(w, r, "/auth/login?next="+url.QueryEscape(r.URL.String()), http.StatusSeeOther)
 		return
···
 
 	rKey := r.PathValue("rkey")
 	inviteToken := r.URL.Query().Get("invite")
+	log.Printf("AcceptInvite: user=%s rkey=%s token=%s", user.ID, rKey, inviteToken[:8]+"...")
 	if inviteToken == "" {
 		http.Error(w, "Invalid invite", http.StatusBadRequest)
 		return
···
 
 	invite, err := collaboration.ValidateInvite(h.DB, inviteToken, rKey)
 	if err != nil {
+		log.Printf("AcceptInvite: validate invite: %v", err)
 		http.Error(w, err.Error(), http.StatusBadRequest)
 		return
 	}
+	log.Printf("AcceptInvite: invite valid, createdBy=%s", invite.CreatedBy)
 
 	// The collaborator's session — needed to get their DID.
 	collabSession, err := h.DB.GetATProtoSession(user.ID)
 	if err != nil || collabSession == nil {
+		log.Printf("AcceptInvite: no ATProto session for collaborator user=%s err=%v", user.ID, err)
 		http.Redirect(w, r, "/auth/atproto?next="+url.QueryEscape(r.URL.String()), http.StatusSeeOther)
 		return
 	}
+	log.Printf("AcceptInvite: collab DID=%s", collabSession.DID)
 
 	// Fetch and update the document from the OWNER's PDS, not the collaborator's.
 	// The invite records the owner's DID in CreatedBy.
···
 		http.Error(w, "Document owner not found", http.StatusInternalServerError)
 		return
 	}
+	log.Printf("AcceptInvite: owner user=%s", ownerUser.ID)
 
 	ownerClient, err := h.xrpcClient(ownerUser.ID)
 	if err != nil {