Centralize durable schema migration ownership

crates/remora-schema/migrations/control/0001_schema_generation.sql

@@ -0,0 +1,8 @@
+CREATE TABLE remora_schema_state (
+    singleton BOOLEAN PRIMARY KEY DEFAULT TRUE CHECK (singleton),
+    generation BIGINT NOT NULL CHECK (generation > 0),
+    updated_at TIMESTAMPTZ NOT NULL DEFAULT now()
+);
+
+INSERT INTO remora_schema_state (singleton, generation)
+VALUES (TRUE, 1);