feat(multiplayer): add shared allocation quota

server/migrations/0007_allocation_quotas.sql

@@ -0,0 +1,10 @@
+-- Optional operator-configured regional spend guard. A missing row means
+-- unlimited, preserving existing deployments until they opt into a quota.
+CREATE TABLE allocation_quotas (
+    region TEXT PRIMARY KEY CHECK (region IN ('EU', 'NA')),
+    window_started_at TIMESTAMPTZ NOT NULL,
+    window_seconds INTEGER NOT NULL CHECK (window_seconds > 0),
+    used_allocations INTEGER NOT NULL DEFAULT 0 CHECK (used_allocations >= 0),
+    max_allocations INTEGER NOT NULL CHECK (max_allocations > 0),
+    updated_at TIMESTAMPTZ NOT NULL
+);