BE-065: POST /activities — save completed activity with GPS route

Track: 1 — MVP
Section: 1.17 GPS Activity Tracking (Running/Cycling)
Status: [ ] Todo
Repo: backend-api
Branch: feature/BE-065-save-activity

Done When

Accepts full activity object including route points array. Validates: distance > 0, duration > 0, route has >=2 points. Calculates avg_pace and avg_speed if not provided. Returns activity with ID

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