initial: Steam-Cloud-style per-user state sync skeleton

HTTP API + on-disk storage + auth-service token verification + dev mode.
31 tests pass, vet clean. See DESIGN.md for the architecture and
README.md for the operator surface.

Pending: pg-backed per-user quota override, snapshot retention / blob GC,
tarball-vs-manifest content cross-check, end-to-end deploy on john.

.gitignore

@@ -0,0 +1,6 @@
+cloud-svc
+data/
+*.test
+*.out
+.idea/
+.vscode/