feat(db): conversation_keys can target user_id alongside legacy device_id

Note: not applied locally — push via pnpm prod:migrate when ready.

supabase/migrations/20260515000002_conversation_keys_user_recipient.sql

@@ -0,0 +1,76 @@
+-- Add per-user recipient/sender columns so conv-keys can be wrapped to a
+-- user identity instead of a specific device. Old per-device columns stay
+-- nullable through the transition; a follow-up migration drops them once
+-- migration telemetry shows >=95% adoption.
+
+alter table public.conversation_keys
+  add column if not exists recipient_user_id uuid references auth.users(id) on delete cascade,
+  add column if not exists sender_user_id    uuid references auth.users(id) on delete restrict;
+
+alter table public.conversation_keys
+  alter column recipient_device_id drop not null,
+  alter column sender_device_id    drop not null;
+
+create unique index if not exists conversation_keys_user_recipient_uniq
+  on public.conversation_keys(conversation_id, recipient_user_id, key_version)
+  where recipient_user_id is not null;
+
+create index if not exists conversation_keys_recipient_user_idx
+  on public.conversation_keys(recipient_user_id);
+
+drop policy if exists conversation_keys_select_owner on public.conversation_keys;
+create policy conversation_keys_select_owner
+  on public.conversation_keys
+  for select
+  to authenticated
+  using (
+    recipient_user_id = auth.uid()
+    or exists (
+      select 1
+        from public.devices d
+       where d.id = recipient_device_id
+         and d.user_id = auth.uid()
+    )
+  );
+
+drop policy if exists conversation_keys_insert_member on public.conversation_keys;
+create policy conversation_keys_insert_member
+  on public.conversation_keys
+  for insert
+  to authenticated
+  with check (
+    exists (
+      select 1
+        from public.conversation_members m
+       where m.conversation_id = conversation_keys.conversation_id
+         and m.user_id = auth.uid()
+         and m.accepted = true
+    )
+    and (
+      sender_user_id = auth.uid()
+      or exists (
+        select 1
+          from public.devices d
+         where d.id = sender_device_id
+           and d.user_id = auth.uid()
+      )
+    )
+    and (
+      (recipient_user_id is not null and exists (
+        select 1
+          from public.conversation_members m
+         where m.conversation_id = conversation_keys.conversation_id
+           and m.user_id = recipient_user_id
+           and m.accepted = true
+      ))
+      or (recipient_device_id is not null and exists (
+        select 1
+          from public.devices d
+          join public.conversation_members m
+            on m.user_id = d.user_id
+           and m.conversation_id = conversation_keys.conversation_id
+         where d.id = recipient_device_id
+           and m.accepted = true
+      ))
+    )
+  );