feat(litellm): front Kimi K3 (Moonshot) as a paid gateway passthrough

Adds model_name kimi-k3 → openai/kimi-k3 @ https://api.moonshot.ai/v1
(OpenAI-compatible), keyed by MOONSHOT_API_KEY (compose env + .env.example
placeholder; real key on server only). Verified live through the gateway.

Two Moonshot constraints captured in the config comment + pinned: K3 accepts
ONLY temperature=1 (else 400), and it is a reasoning model (CoT in
reasoning_content, answer in content — needs adequate max_tokens or content
returns empty). Model id confirmed via /v1/models.

stacks/litellm/compose.yaml

@@ -47,8 +47,10 @@ services:
       # leave blank; LiteLLM still needs the var to exist).
       - VLLM_API_KEY=${VLLM_API_KEY:-}
       # Cloud API keys fronted by the gateway for unified logging (z.ai GLM,
-      # etc.). Paid — only gateway-keyed callers reach them, but they spend.
+      # Moonshot Kimi, etc.). Paid — only gateway-keyed callers reach them,
+      # but they spend.
       - Z_AI_API_KEY=${Z_AI_API_KEY:-}
+      - MOONSHOT_API_KEY=${MOONSHOT_API_KEY:-}
       # Langfuse-ready: blank until you bolt Langfuse on. Filling these +
       # uncommenting the callback in config.yaml is the entire upgrade.
       - LANGFUSE_PUBLIC_KEY=${LANGFUSE_PUBLIC_KEY:-}