feat: anthropic cache support — preserve cache_control, forward anthropic-version header, report real usage tokens

- Add anthropicPassthrough flag to BackendConfig for native Anthropic backends
- Preserve cache_control on content blocks and system prompt (keep structured)
- Extract and forward anthropic-version header from client to backend
- Report actual token usage from backend response (input_tokens, output_tokens)
- Support native Anthropic passthrough (no translation) for compatible backends
- Wire anthropic-version through all entry points: index.ts, router.ts, worker.ts

src/lib/router.ts

@@ -478,7 +478,8 @@ export async function handleRequest(
 		try {
 			const body = await req.json();
 			const sessionId = crypto.randomUUID();
-			return handleAnthropicMessages(body, proxyPool!, sessionPool!, sessionId);
+			const anthropicVersion = req.headers.get("anthropic-version") ?? undefined;
+			return handleAnthropicMessages(body, proxyPool!, sessionPool!, sessionId, undefined, anthropicVersion);
 		} catch {
 			return new Response(
 				JSON.stringify({