feat(polar): sync Polar exercises into Workout rows

Pull the athlete's exercises from the stateless GET /v3/exercises list (last
30 days of uploads, no transaction, no userId in the path) and upsert each into
the Workout table as source POLAR, keyed on (userId, source, externalId) so a
re-fetch or Polar-side re-analysis overwrites in place. A new table-driven
sport map normalises the raw Polar sport labels into the canonical
WorkoutSportType bucket at ingest, mirroring the Strava/WHOOP mappers; the raw
labels and training load ride metadata for provenance.

The exercise leg rides the existing hourly polar-sync cohort tick alongside the
vitals leg, with independent attribution on the shared polar ledger: each leg
is attempted and records its own failure, and neither can mask the other's
result. Polar takes its place in the workout source ladder between Withings and
Strava. No migration and no OpenAPI change: MeasurementSource.POLAR and the
full workouts source enum already exist.

Refs #568

package.json

@@ -1,6 +1,6 @@
 {
   "name": "healthlog",
-  "version": "1.32.14",
+  "version": "1.32.15",
   "description": "Self-hosted personal-health-tracking PWA with Withings integration, AI insights, and doctor-report PDF export.",
   "license": "PolyForm-Noncommercial-1.0.0",
   "homepage": "https://healthlog.dev",