feat: add Anthropic-compatible AI proxy endpoint

- Create src/lib/anthropic-proxy.ts: accepts Anthropic Messages API
  format (POST /v1/messages) and routes to the same backend providers
- Anthropic model names (claude-sonnet-4, claude-3-haiku, claude-opus-4)
  map to backend models with full request/response translation
- Streaming (SSE) via Anthropic protocol: message_start,
  content_block_delta, message_stop events
- Add /v1/messages route to server with CORS and proxy pool fallback
- Reuses MODEL_ROUTES from ai-proxy.ts for consistent backend routing

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/lib/ai-proxy.ts

@@ -46,7 +46,8 @@ export interface BackendConfig {
 
 // ─── Model routing table ─────────────────────────────────────────────────────────
 
-const MODEL_ROUTES: Record<string, BackendConfig> = {
+/** Map of model name → backend configuration. Exported for reuse by anthropic-proxy. */
+export const MODEL_ROUTES: Record<string, BackendConfig> = {
 	// ── opencode.ai (OpenAI-compatible — passthrough) ────────────────
 	"deepseek-v4-flash-free": {
 		provider: "opencode",