Public plugin routes: opt-in Cache-Control for GET responses

[swissky]

Problem

All plugin API responses are served with Cache-Control: private, no-store — the right default for authenticated data. But public plugin routes (public: true) often serve identical data to every visitor: a catalog listing, a public search endpoint, a widget feed. Today every page view pays a full round-trip to the Worker and D1, even though the response is the same for everyone and changes rarely. On Cloudflare that's exactly the workload the CDN cache exists for.

Proposal

An opt-in per-route header value, additive and default-off:

routes: {
	catalog: {
		public: true,
		cacheControl: "public, max-age=60, stale-while-revalidate=300",
		handler: async (ctx) => listProducts(ctx),
	},
},

Guardrails, so this can't leak private data:

• Honored only on routes that are also public: true — the metadata layer never exposes cacheControl for private routes, so authenticated responses always keep private, no-store even if a route sets both flags.
• Applied only to successful GET/HEAD responses; errors and other methods keep the default.
• The plugin author supplies the literal header value — no caching DSL to maintain, and existing routes are untouched.

I have a working implementation with tests and docs ready to PR if this direction sounds right.