Apply /simplify: share META_PREFIX/TARBALL_PREFIX with the key builders

The expiry sweep hardcoded ['meta/', 'tarball/'] kept in sync with metaKey/
tarballKey only by a 'must match' comment; a prefix rename would silently desync
the cron (it would stop matching and R2 would leak with no error). Export the two
prefixes from r2Cache, use them in both the key builders and the sweep.

Author: fengmk2

src/cache/r2Cache.ts

@@ -1,8 +1,13 @@
 /** R2 object keys and the public URL for a generated preview build. */
 import type { Env } from '../config'
 
+// Per-version object key prefixes. Shared with the expiry sweep
+// (cleanupExpiredArtifacts), so a rename here can't silently desync it.
+export const TARBALL_PREFIX = 'tarball/'
+export const META_PREFIX = 'meta/'
+
 export function tarballKey(name: string, version: string): string {
-  return `tarball/${name}/${version}.tgz`
+  return `${TARBALL_PREFIX}${name}/${version}.tgz`
 }
 
 /** Public URL of a preview tarball (the inverse of parseTarballPath). */
@@ -15,7 +20,7 @@ export function tarballUrl(
 }
 
 export function metaKey(name: string, version: string): string {
-  return `meta/${name}/${version}.json`
+  return `${META_PREFIX}${name}/${version}.json`
 }
 
 /**

src/preview/cleanupExpired.ts

@@ -1,10 +1,11 @@
 import type { Env } from '../config'
+import { META_PREFIX, TARBALL_PREFIX } from '../cache/r2Cache'
 import { REF_TTL_MS } from './getConfiguredRefs'
 
-// R2 key prefixes for the per-version preview artifacts. Must match `metaKey` /
-// `tarballKey` in ../cache/r2Cache. NOT `meta-index/` or `refs/`: those are the
-// live indexes (self-pruned on write), not per-version orphans.
-const ARTIFACT_PREFIXES = ['meta/', 'tarball/']
+// The per-version artifact prefixes (shared with the key builders). NOT
+// `meta-index/` or `refs/`: those are the live indexes (self-pruned on write),
+// not per-version orphans, and their prefixes are disjoint from these.
+const ARTIFACT_PREFIXES = [META_PREFIX, TARBALL_PREFIX]
 
 /**
  * Delete per-version preview artifacts (meta + tarball) whose ref TTL has lapsed,