fix(submit): 审阅绑定上下文 sha256，批准须带摘要，构建只从内部 API 取上下文并校验字节

internal/store/migrations/0025_context_digest.sql

@@ -0,0 +1,12 @@
+-- Binds a submission's review to the exact bytes that get built. The upload
+-- records the sha256 of the stored context tarball; an admin approves naming
+-- the digest they reviewed, and the approve CAS only wins while the row still
+-- carries it. A re-upload while pending replaces both the blob and the digest,
+-- so an approval issued against the old digest loses instead of building
+-- content nobody saw. NULL marks a context uploaded before digests were kept
+-- (or no upload yet); such a row cannot be approved until it is uploaded again.
+ALTER TABLE image_submissions ADD COLUMN context_sha256 text;
+
+-- The digest the build was allowed to consume. The context-fetch step refuses
+-- any other bytes, so a context swapped after approval fails the build.
+ALTER TABLE image_builds ADD COLUMN context_digest text;