feat(P5A.T1): conversation_watch_sessions table with RLS + realtime

supabase/migrations/20260516000008_watch_together.sql

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+-- Phase 5A: per-conversation synchronized YouTube playback.
+--
+-- conversation_watch_sessions: one row per Watch-Together session. The bubble
+-- in the chat is a normal `messages` row whose plaintext payload is
+-- `{v:1, type:'watch_together', session_id:<id>}`. The owner's player drives
+-- current_state (jsonb {playing, position_seconds, updated_at_ms}); other
+-- joiners reconcile via realtime postgres_changes UPDATE when local drift
+-- exceeds 2 seconds.
+
+create table if not exists public.conversation_watch_sessions (
+  id              uuid primary key default gen_random_uuid(),
+  conversation_id uuid not null references public.conversations(id) on delete cascade,
+  owner_user_id   uuid not null references auth.users(id) on delete cascade,
+  video_id        text not null,
+  started_at      timestamptz not null default now(),
+  ended_at        timestamptz null,
+  current_state   jsonb not null default '{"playing":false,"position_seconds":0,"updated_at_ms":0}'::jsonb,
+  constraint conversation_watch_sessions_video_id_len check (length(video_id) between 1 and 64)
+);
+
+create index if not exists conversation_watch_sessions_conv_idx
+  on public.conversation_watch_sessions(conversation_id, started_at desc);
+
+alter table public.conversation_watch_sessions enable row level security;
+
+drop policy if exists conversation_watch_sessions_select on public.conversation_watch_sessions;
+drop policy if exists conversation_watch_sessions_insert on public.conversation_watch_sessions;
+drop policy if exists conversation_watch_sessions_update on public.conversation_watch_sessions;
+
+create policy conversation_watch_sessions_select
+  on public.conversation_watch_sessions
+  for select
+  using (public.is_conversation_member(conversation_id));
+
+create policy conversation_watch_sessions_insert
+  on public.conversation_watch_sessions
+  for insert
+  with check (
+    public.is_conversation_member(conversation_id)
+    and owner_user_id = auth.uid()
+  );
+
+create policy conversation_watch_sessions_update
+  on public.conversation_watch_sessions
+  for update
+  using (owner_user_id = auth.uid())
+  with check (owner_user_id = auth.uid());
+
+alter table public.conversation_watch_sessions replica identity full;
+
+do $$
+begin
+  if not exists (
+    select 1
+      from pg_publication_tables
+      where pubname = 'supabase_realtime'
+        and schemaname = 'public'
+        and tablename = 'conversation_watch_sessions'
+  ) then
+    execute 'alter publication supabase_realtime add table public.conversation_watch_sessions';
+  end if;
+end
+$$;