fix(sync): support new Garmin API metric format + backfill script + make targets

[gerfru]

Summary

• Garmin API format change: activityDetailMetrics changed from per-metric-type groups (with sequenceNumber) to positional value arrays per timepoint (with a separate metricDescriptors lookup). The old mapper silently returned 0 records for all activities using the new format.
• Elevation fallback removed: directGrade is a % slope value, not elevation in metres — was silently writing wrong data
• Backfill script: one-shot make backfill-records to retroactively fetch GPS/HR detail for historical activities missing records
• Makefile fixes: make trigger-sync and make db were reading DB_APP_USER from the wrong env file, causing role "-d" does not exist errors

Changes

File	Change
mapper.py	_build_metric_index_v2 for new format; auto-detects via metricDescriptors key; removes directGrade elevation fallback
repositories/base.py	Abstract get_activities_without_records method
repositories/timescale.py	Implementation: SELECT activities WHERE no records exist
backfill_records.py	New script: fetches Garmin detail for all activities with 0 records, rate-limited at 0.5 s/call
Makefile	backfill-records target; hardcode garmin_app in trigger-sync + db targets
tests/test_mapper.py	5 tests for new positional array format (HR, elevation, pace, GPS, count)
tests/test_repository.py	2 tests for get_activities_without_records

Test plan

• cd sync-service && .venv/bin/pytest tests/ -v — all tests pass
• make trigger-sync — no longer errors with role "-d" does not exist
• make backfill-records — fetches records for outdoor activities missing GPS/HR data
• Activity page for a cycling/running activity shows HR + elevation + pace charts after backfill

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