Record collaboration row when invite is accepted

Co-Authored-By: Claude Sonnet 4.6 <[e-mail]>

internal/handler/handler.go

···
 	// Already a collaborator — redirect to the owner-scoped URL.
 	for _, c := range doc.Collaborators {
 		if c == collabSession.DID {
+			h.DB.AddCollaboration(collabSession.DID, invite.CreatedBy, rKey) // ensure row exists
 			http.Redirect(w, r, "/docs/"+invite.CreatedBy+"/"+rKey, http.StatusSeeOther)
 			return
 		}
···
 	}
 
 	h.DB.DeleteInvite(invite.Token)
+
+	if err := h.DB.AddCollaboration(collabSession.DID, invite.CreatedBy, rKey); err != nil {
+		log.Printf("AcceptInvite: record collaboration: %v", err)
+		// Non-fatal — collaborator is already on the document, just redirect.
+	}
 
 	// Redirect to owner-scoped document URL so the view handler knows whose PDS to query.
 	http.Redirect(w, r, "/docs/"+invite.CreatedBy+"/"+rKey, http.StatusSeeOther)