src: use workers tiered cache

[flakey5]

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[cursor]

PR Summary

Medium Risk
Caching behavior for dist/download/docs/api traffic changes at the edge; incorrect cache headers or tags could serve stale assets until purge, though existing cache-control rules remain in place.

Overview
Replaces in-worker GET response caching (CacheMiddleware + CACHING env) with Cloudflare Workers tiered cache enabled in wrangler.jsonc for staging and prod.

GET routes now hit R2Middleware directly (same as HEAD), and successful responses add Cache-Tag values (release-worker:file, :directory, :origin) so edge cache can be targeted for purges. Architecture docs are updated to show caching at Cloudflare and to drop the cache middleware from the worker flow.

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Cursor Bugbot has reviewed your changes using default effort and found 2 potential issues.

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