feat(litellm): add Redis for router state/rate-limits/budgets/cache

New redis service (redis:7-alpine, password-protected, no persistence
volume — everything it holds is cache/coordination state). litellm gets
REDIS_HOST/REDIS_PORT/REDIS_PASSWORD, which is all LiteLLM needs to use it
for router state, rate limits/budgets, and cache invalidation — no
litellm-config.yaml block required (docs.litellm.ai/docs/proxy/caching).
REDIS_PASSWORD added to .env.example and update.sh's auto-generated
secrets.

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

litellm-config.yaml

@@ -56,8 +56,9 @@ router_settings:
   # ponytail: LiteLLM's request-prioritization scheduler is beta (see
   # docs/proxy-request-priority.md) — exact settings key/shape must be
   # confirmed against LiteLLM's current docs and smoke-tested against
-  # llama.cpp before workloads depend on it. Single-instance deployment,
-  # no Redis configured — add one only if the scheduler turns out to need it.
+  # llama.cpp before workloads depend on it. Redis is available (see the
+  # litellm service's REDIS_* env vars in docker-compose.yml) if the
+  # scheduler needs shared state for it.
   enable_priority_scheduling: true
 
 general_settings: