ci(nightly): switch from daily to on-change + weekly schedule

Reduce GitHub Actions minutes consumption on the Free plan.
The nightly was running 3 jobs (full-live, slow-live, arm64-smoke)
every day at 04:23 UTC regardless of code changes, consuming
~1,590 min/month (80% of the 2,000 min quota).

New triggers:
- Weekly (Sunday 04:23 UTC): catches environment drift (base image
  updates, dependency changes) even without code changes
- On push to main/dev: runs full test coverage on every real change
- Manual dispatch: unchanged

The CI workflow (ci.yml) continues to handle PR/push feedback with
lighter tests. This nightly adds the full live_api suite, slow tests,
and unconditional ARM smoke that CI excludes.

Projected savings: ~1,590 → ~320 min/month.

Generated with [Claude Code](https://claude.ai/code)
via [Happy](https://happy.engineering)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

Author: 3 people

.github/workflows/nightly.yml

@@ -2,7 +2,11 @@ name: Nightly Validation
 
 on:
   schedule:
-    - cron: '23 4 * * *'
+    # Weekly full validation: Sunday 04:23 UTC (catches environment drift)
+    - cron: '23 4 * * 0'
+  push:
+    # On-change: run full suite on every push to main/dev
+    branches: [main, dev]
   workflow_dispatch:
 
 concurrency: