Enable beta auto-deploy on master; manual prod button (#26) (#30)

## Summary

Closes #26.

- Replace auto-prod `deploy.yml` with `deploy-beta.yml` (push to `master` → tests → `--env beta`) and `deploy-prod.yml` (`workflow_dispatch` only → `--env prod`), matching `dta_service` / `chat_web_app`
- Default `ALLOWED_HOSTS` / `CORS_ALLOWED_ORIGINS` include beta API + SPA hosts so beta frontend (and Tianji-instrumented pages) can call the API
- Expand `.env.prod.example` beta block (CORS, CSRF, OAuth callbacks, `FRONTEND_BASE_URL`, Tianji note)
- Harden `validate-env.sh` for beta secrets; README documents beta auto / prod button

## Tianji

Backend does not load `tracker.js`. FE already owns wiring (`chat_web_app#35`, closed).

- Beta SPA origin: `https://beta.chat.aimloperations.com`
- Beta Tianji website ID (FE `.env.beta`): `cms38bw671mf9n5jjw3xp1j3q`

## Coordination

- Infra: [server-infra#7](ai_ml_operations/server-infra#7) (`host_apps` beta `:8013`, `chat_backend_beta` DB, secret, NPM)
- FE companion: [chat_web_app#35](ai_ml_operations/chat_web_app#35)

## Test plan

- [ ] PR CI / unit tests green
- [ ] Merge to `master` triggers **Deploy Beta** only (not prod)
- [ ] Manual **Deploy Prod** `workflow_dispatch` still deploys `--env prod`
- [ ] After infra#7: beta container healthy on **8013** with `chat_backend_beta`
- [ ] Beta hosts / CSRF / CORS allow `https://beta.chat.aimloperations.com`Reviewed-on: #30

scripts/validate-env.sh

@@ -44,9 +44,11 @@ if ((${#missing[@]} > 0)); then
   exit 1
 fi
 
-if [[ "$DJANGO_ENV" == "prod" && ( "$DJANGO_SECRET_KEY" == change-me* || "$DJANGO_SECRET_KEY" == *dev-only* ) ]]; then
-  echo "DJANGO_SECRET_KEY must be changed from the example value for production." >&2
-  exit 1
+if [[ "$DJANGO_ENV" == "prod" || "$DJANGO_ENV" == "beta" ]]; then
+  if [[ "$DJANGO_SECRET_KEY" == change-me* || "$DJANGO_SECRET_KEY" == *dev-only* || "$DJANGO_SECRET_KEY" == django-insecure* ]]; then
+    echo "DJANGO_SECRET_KEY must be a real secret for $DJANGO_ENV (not an example/dev value)." >&2
+    exit 1
+  fi
 fi
 
 echo "Environment validation passed (DJANGO_ENV=$DJANGO_ENV)."