Codify Cloud Run scaling settings into deploy config (warm floor + max-instances 8)

[anth-volk]

Fixes #3767

Why

Two production Cloud Run scaling settings were being managed by hand and were not reflected in the repo:

1. Service-level warm floor (minScale=2). Set manually via gcloud --min during the PR 4 host cutover; it lived only as live service state. It survives CI deploys today only because the deploy never asserts a service-level min — invisible to anyone reading the repo, and one --min 0 or service recreation away from silently vanishing. Overnight sweeps confirm this floor is what holds the service at 2 warm instances through the bot trough with zero cold-start churn.
2. max-instances 4. At the current 50/50 LB split the service already sits pinned at the ceiling of 4 during the daytime peak (instance_count flatlines at 4.0). The imminent 100% Cloud Run cutover roughly doubles load, so 4 would bottleneck.

What

• New CLOUD_RUN_SERVICE_MIN_INSTANCES env (default 0 in cloud_run_env.sh; prod 2, staging 0 in push.yml). The deploy now passes --min ${CLOUD_RUN_SERVICE_MIN_INSTANCES} — the service-level floor — on every deploy.
• Revision-level --min-instances 0 unchanged. A positive revision-level min is baked immutably onto every new revision, so each tagged no-traffic candidate would pin its own warm 16Gi instance (per-tag cost bomb). The existing invariant test guarding this stays green.
• max-instances 4 → 8 in the production job.
• Runbook (docs/migration/cloud-run-operations.md) updated: warm floor is now applied by CI, not manually-once; max sizing rationale recorded.
• Deploy-script unit tests extended (service-level --min emitted, prod max 8, per-job service-min values); changelog fragment added.

Live state

The live production service was already raised to max-instances 8 (new revision 00045-2rd, serving 100%) and carries minScale 2 out-of-band, so production matches this config now. This PR makes both durable across future releases — the production Cloud Run deploy only runs on the release commit, so without it the next release would revert max to 4.

Testing

.venv/bin/python -m pytest tests/unit/test_cloud_run_deploy_scripts.py -q
34 passed

Dry-run confirms the emitted command: --min-instances 0 --min 2 --max-instances 8.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.03%. Comparing base (e004986) to head (e287591).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3768   +/-   ##
=======================================
  Coverage   80.03%   80.03%           
=======================================
  Files          70       70           
  Lines        4342     4342           
  Branches      809      809           
=======================================
  Hits         3475     3475           
  Misses        648      648           
  Partials      219      219           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.