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>

.env.example

@@ -39,6 +39,9 @@ LITELLM_PORT=4000
 LITELLM_MASTER_KEY=
 LITELLM_SALT_KEY=
 LITELLM_DB_PASSWORD=
+# Backs litellm's router state/rate-limits/budgets/cache invalidation
+# (the redis service). Random value, filled in automatically — leave blank.
+REDIS_PASSWORD=
 # Admin UI login (https://<proxy>/ui). Without these, LiteLLM falls back to
 # username "admin" / password = LITELLM_MASTER_KEY — set these instead so the
 # master key never has to be typed into the browser. UI_PASSWORD is filled