feat(multiplayer): persist connection generation leases

server/migrations/0011_connection_leases.sql

@@ -0,0 +1,17 @@
+ALTER TABLE match_participants
+    ADD COLUMN disconnected_at TIMESTAMPTZ;
+
+-- The pre-lease connection receipt populated connected_at but did not advance
+-- the already-present generation column. Preserve those live admissions as
+-- generation one before enforcing the lease invariant.
+UPDATE match_participants
+SET connection_generation = 1
+WHERE connected_at IS NOT NULL AND connection_generation = 0;
+
+ALTER TABLE match_participants
+    ADD CONSTRAINT match_participants_connection_lease
+    CHECK (
+        (connection_generation = 0 AND connected_at IS NULL AND disconnected_at IS NULL)
+        OR
+        (connection_generation > 0 AND connected_at IS NOT NULL)
+    ) NOT VALID;

server/migrations/down/0011_connection_leases.sql

@@ -0,0 +1,3 @@
+ALTER TABLE match_participants
+    DROP CONSTRAINT IF EXISTS match_participants_connection_lease,
+    DROP COLUMN IF EXISTS disconnected_at;