feat(litellm): front z.ai GLM cloud models through the gateway

Adds explicit gateway entries for the four z.ai GLM models (glm-5.1,
glm-5-turbo, glm-4.7, glm-4.5-air) routed to api.z.ai with Z_AI_API_KEY,
plus the compose env passthrough + .env.example doc. Explicit entries
win over the llama-swap wildcard (distinct IDs, no collision). Extends
the gateway's unified logging to cloud inference, not just local
vLLM/llama-swap.

Cost note: paid API — only gateway-keyed callers reach these, but calls
spend z.ai credits (documented in config + compose comments).

stacks/litellm/compose.yaml

@@ -42,6 +42,9 @@ services:
       # Upstream vLLM API key (empty in the vllm stack's .env by default →
       # leave blank; LiteLLM still needs the var to exist).
       - VLLM_API_KEY=${VLLM_API_KEY:-}
+      # Cloud API keys fronted by the gateway for unified logging (z.ai GLM,
+      # etc.). Paid — only gateway-keyed callers reach them, but they spend.
+      - Z_AI_API_KEY=${Z_AI_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:-}