Centralize durable schema migration ownership

crates/remora-schema/migrations/worker/0001_descriptors.sql

@@ -0,0 +1,14 @@
+CREATE TABLE worker_descriptors (
+    descriptor_id TEXT PRIMARY KEY,
+    attempt_id TEXT NOT NULL REFERENCES attempts (attempt_id) ON DELETE CASCADE,
+    kind TEXT NOT NULL CHECK (kind IN ('resolved_source', 'checkpoint')),
+    schema_version INTEGER NOT NULL CHECK (schema_version >= 1),
+    payload JSONB NOT NULL,
+    byte_length BIGINT NOT NULL CHECK (byte_length >= 0),
+    item_count BIGINT CHECK (item_count >= 0),
+    checksum TEXT NOT NULL,
+    created_at TIMESTAMPTZ NOT NULL
+);
+
+CREATE INDEX worker_descriptors_attempt_idx
+    ON worker_descriptors (attempt_id, kind, created_at, descriptor_id);