feat(multiplayer): lease allocating match claims

server/migrations/0006_match_allocation_claims.sql

@@ -0,0 +1,12 @@
+-- Allocation is an external call, so a durable leased claim fences competing
+-- allocator replicas before any provider request. A timed-out claim can be
+-- recovered with the same deterministic allocation ID after a worker crash.
+ALTER TABLE matches
+    ADD COLUMN allocation_id TEXT UNIQUE,
+    ADD COLUMN allocation_claimed_at TIMESTAMPTZ,
+    ADD CONSTRAINT matches_allocation_claim_pair
+        CHECK ((allocation_id IS NULL) = (allocation_claimed_at IS NULL));
+
+CREATE INDEX matches_allocating_claimable
+    ON matches (created_at, match_id)
+    WHERE state = 'ALLOCATING' AND server_id IS NULL;