Show mobile build slug separately from the church seen in analytics

[redreceipt]

Summary

• Distinguish a mobile build's configured slug (buildChurch from Platforms) from the church selected inside the app. The two differ in multi-church apps such as Apollos Preview.
• Use the build slug if present on any recent event for the bundle; for legacy events without it, say Unknown build slug instead of attributing the build to apollos_demo. Continue displaying the observed church separately. Other platforms remain labeled “Church seen.”
• Depends on ApollosProject/apollos-platforms#7464. Keep this PR draft until that change is merged and telemetry reaches Segment. No historical bundle-to-church mapping is invented.

Validation

• 215 unit tests, Ruff check/format, focused mypy pass. Whole-repo mypy is blocked by pre-existing venv/bin/activate_this.py:58 error; not in this diff.

Proof

• Local HTTP /apps 200 response from the Flask app using a disposable BigQuery client outside Git and the real public US Apple lookup. The response shows apollos_preview as build slug alongside “Church seen: apollos_demo”, and an older row without build metadata says “Unknown build slug”. This proves rendering and query construction, not that deployed apps already emit the new field. Screenshot attached below.