feat(ci): missed-run heartbeat for the daily sync cron

[timon0305]

Closes #71

Problem

The daily translation sync (.github/workflows/sync-translation.yml, cron: "0 0 * * *") surfaces a failed run on its own, but a run that never fires is silent: GitHub can drop scheduled runs, auto-disable the workflow after repo inactivity, or lose the schedule. A silently missed sync can go unnoticed until translations fall behind.

What changed

• New scheduled workflow .github/workflows/heartbeat.yml (0 12 * * *, offset from the sync's 00:00) queries the last successful scheduled sync-translation run via the gh API and alerts when it is older than the threshold. It runs independently, so it still fires once the sync workflow has stopped.
• The staleness decision is a pure heartbeat_run_is_stale() in assets/lib.sh (missing or older-than-threshold), with four bats tests (no-run / older / within / boundary).
• HEARTBEAT_MAX_AGE_HOURS = 30 in assets/env.sh carries margin above the 24h cadence, so a delayed run does not trip the alert while a fully missed day is caught by the next heartbeat.

Alerting is interim: the job fails so the missed run surfaces to watchers. It is a one-step swap to the shared notification channel once the failure-notification work (#66) provides one.

Repo lint (shellcheck + actionlint + JSON-schema) is clean and the full bats suite passes locally (135).

Summary by CodeRabbit

• New Features
  • Added a daily synchronization “heartbeat” check to detect missed or paused scheduled translation runs.
  • Supports manual triggering of the heartbeat check.
  • Alerts and fails the workflow when the most recent successful scheduled run is older than the allowed threshold.
• Bug Fixes
  • Uses the original run time to detect staleness more reliably, so reruns of old runs don’t hide a stopped schedule.
• Tests
  • Added Bats coverage for missing, stale, fresh, and exact-threshold heartbeat behavior.

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 0093fc83-90b5-4359-9ef0-7ae430f2c47d

📥 Commits

Reviewing files that changed from the base of the PR and between 746027d and af97b90.

📒 Files selected for processing (1)

• .github/workflows/heartbeat.yml

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📝 Walkthrough

Walkthrough

Adds a scheduled heartbeat workflow that checks the latest successful scheduled translation-sync run, uses shared timestamp validation, and fails with an error when the run exceeds the 30-hour threshold. Tests cover missing, stale, fresh, and boundary timestamps.

Changes

Translation sync heartbeat

Layer / File(s)	Summary
Heartbeat constants and stale-run validation .github/workflows/assets/env.sh, .github/workflows/assets/lib.sh, tests/test_lib.bats	Adds the heartbeat phase and 30-hour threshold, implements stale timestamp detection, and tests missing, stale, fresh, and exact-threshold behavior.
Scheduled heartbeat workflow .github/workflows/heartbeat.yml	Runs daily or manually, finds the latest successful scheduled sync run using createdAt, validates its age, emits an error for stale state, and closes the phase through traps.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Sequence Diagram(s)

sequenceDiagram
  participant HeartbeatWorkflow
  participant GitHubActions
  participant heartbeat_run_is_stale
  HeartbeatWorkflow->>GitHubActions: query latest successful scheduled sync run
  GitHubActions-->>HeartbeatWorkflow: return createdAt timestamp
  HeartbeatWorkflow->>heartbeat_run_is_stale: compare timestamp with 30-hour threshold
  heartbeat_run_is_stale-->>HeartbeatWorkflow: return stale or fresh status
  HeartbeatWorkflow-->>GitHubActions: emit error and fail when stale

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🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Linked Issues check	⚠️ Warning	The heartbeat and threshold logic satisfy #71, but it does not use the shared notification channel required by the issue.	Route heartbeat alerts through the shared failure-notification channel instead of only failing the job, or note the dependency if unavailable.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the new missed-run heartbeat workflow for the daily sync cron.
Out of Scope Changes check	✅ Passed	All changes support the heartbeat workflow, helper logic, and tests for the missed-run detection goal.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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