perf: sync protocol and core hardening updates

deploy/migrations/0063_channel_update_retention.up.sql

@@ -0,0 +1,35 @@
+-- Channel-scoped update retention checkpoint.
+--
+-- channel_update_events may now be pruned in bounded per-channel transactions. The checkpoint is
+-- the durable protocol boundary: request pts below retained_through_pts must receive
+-- updates.channelDifferenceTooLong; latest_event_date/latest_pts remain after row deletion so an
+-- account-level updates.getDifference can still emit UpdateChannelTooLong for an offline member.
+CREATE TABLE IF NOT EXISTS public.channel_update_checkpoints (
+    channel_id bigint PRIMARY KEY REFERENCES public.channels(id) ON DELETE CASCADE,
+    retained_through_pts integer NOT NULL DEFAULT 0 CHECK (retained_through_pts >= 0),
+    latest_event_date integer NOT NULL DEFAULT 0 CHECK (latest_event_date >= 0),
+    latest_pts integer NOT NULL DEFAULT 0 CHECK (latest_pts >= 0),
+    updated_at timestamp with time zone NOT NULL DEFAULT now(),
+    CONSTRAINT channel_update_checkpoints_floor_check CHECK (retained_through_pts <= latest_pts)
+);
+
+-- Backfill every existing channel, including channels without an event row, so read paths can use
+-- the checkpoint directly without a full event-log fallback.
+INSERT INTO public.channel_update_checkpoints (
+    channel_id, retained_through_pts, latest_event_date, latest_pts
+)
+SELECT c.id,
+       0,
+       COALESCE(MAX(e.date), 0)::integer,
+       c.pts
+FROM public.channels c
+LEFT JOIN public.channel_update_events e ON e.channel_id = c.id
+GROUP BY c.id, c.pts
+ON CONFLICT (channel_id) DO UPDATE SET
+    latest_event_date = GREATEST(channel_update_checkpoints.latest_event_date, EXCLUDED.latest_event_date),
+    latest_pts = GREATEST(channel_update_checkpoints.latest_pts, EXCLUDED.latest_pts),
+    updated_at = now();
+
+-- Global retention candidate seek. Exact per-channel deletion continues to use PK(channel_id,pts).
+CREATE INDEX IF NOT EXISTS channel_update_events_retention_seek_idx
+    ON public.channel_update_events (date, channel_id, pts);