feat: support ANTHROPIC_BASE_URL and ANTHROPIC_AUTH_TOKEN for custom endpoints

- apps/server/src/providers/claude-provider.ts

Add ANTHROPIC_BASE_URL and ANTHROPIC_AUTH_TOKEN to the environment variable
allowlist, enabling use of LLM gateways (LiteLLM, Helicone) and Anthropic-
compatible providers (GLM 4.7, Minimax M2.1, etc.).

Closes #338

apps/server/src/providers/claude-provider.ts

@@ -19,6 +19,8 @@ import type {
 // Only these vars are passed - nothing else from process.env leaks through.
 const ALLOWED_ENV_VARS = [
   'ANTHROPIC_API_KEY',
+  'ANTHROPIC_BASE_URL',
+  'ANTHROPIC_AUTH_TOKEN',
   'PATH',
   'HOME',
   'SHELL',