Prevent sharing a diagram with yourself

RLS is the real guard (diagram_collaborators_insert/update now reject
user_id = the diagram's owner, regardless of who's performing the write —
covers a Super Admin acting on someone else's diagram too, not just the
normal owner path); the client-side check in
SupabaseDiagramCollaboratorRepository.add is just there to surface a
friendly message instead of the raw 42501.

Verified: tsc -b and oxlint clean; supabase db reset + 53/53 pgTAP tests
pass (1 new test).

supabase/migrations/20260914000000_prevent_self_collaborator.sql

@@ -0,0 +1,29 @@
+-- Prevents a diagram's owner from ending up as their own collaborator row
+-- (found via manual testing: the UI let you "share" a diagram with
+-- yourself). Ownership already implies full access, so a self-collaborator
+-- row is never meaningful — enforced here, not just in the client, since
+-- the client-side check alone wouldn't stop a Super Admin's "add on behalf
+-- of the owner" path or any other direct API access from creating one.
+
+drop policy "diagram_collaborators_insert" on public.diagram_collaborators;
+
+create policy "diagram_collaborators_insert" on public.diagram_collaborators for insert
+  with check (
+    (public.is_super_admin() or public.diagram_owner_id(diagram_id) = auth.uid())
+    and user_id != public.diagram_owner_id(diagram_id)
+  );
+
+-- The existing update policy had no WITH CHECK at all (only USING) — added
+-- here too, defensively, in case user_id (part of the primary key) is ever
+-- changed via UPDATE rather than delete+insert.
+drop policy "diagram_collaborators_update" on public.diagram_collaborators;
+
+create policy "diagram_collaborators_update" on public.diagram_collaborators for update
+  using (
+    public.is_super_admin()
+    or public.diagram_owner_id(diagram_id) = auth.uid()
+  )
+  with check (
+    (public.is_super_admin() or public.diagram_owner_id(diagram_id) = auth.uid())
+    and user_id != public.diagram_owner_id(diagram_id)
+  );

supabase/tests/rls.sql

@@ -25,7 +25,7 @@ begin;
 
 create extension if not exists pgtap with schema extensions;
 
-select plan(52);
+select plan(53);
 
 -- ----------------------------------------------------------------------
 -- Fixtures (as postgres — RLS does not apply)
@@ -138,6 +138,13 @@ select throws_ok(
 
 select set_config('request.jwt.claim.sub', '11111111-1111-1111-1111-111111111111', true);
 
+select throws_ok(
+  $$ insert into public.diagram_collaborators (diagram_id, user_id, permission)
+     values ('b0000000-0000-0000-0000-000000000001', '11111111-1111-1111-1111-111111111111', 'edit') $$,
+  '42501'::char(5), null,
+  'alice (owner) cannot add herself as a collaborator on her own diagram'
+);
+
 select lives_ok(
   $$ insert into public.diagram_collaborators (diagram_id, user_id, permission)
      values ('b0000000-0000-0000-0000-000000000001', '22222222-2222-2222-2222-222222222222', 'view') $$,