refactor: merge internal/sync into internal/garmin, regroup api files and routes

Garmin auth/sync routes move under /api/garmin/*; sync.Service becomes
garmin.Sync with garmin.SyncConfig/ClientConfig; applog becomes
internal/log; the test mock moves into the garmin package as MockClient
(breaking the test-only import cycle the merge created); stale test
URLs and type names updated to match.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/cmd/seedsample/main.go

@@ -13,9 +13,8 @@ import (
 	"time"
 
 	"geniusrun/backend/internal/classify"
-	"geniusrun/backend/internal/garmin/mock"
+	"geniusrun/backend/internal/garmin"
 	"geniusrun/backend/internal/store"
-	appsync "geniusrun/backend/internal/sync"
 )
 
 func main() {
@@ -73,8 +72,8 @@ func main() {
 		IsActive: true,
 	}))
 
-	m := &mock.Client{}
-	svc := appsync.NewService(m, db, userID, appsync.Config{MinConfidence: 0.6}, nil)
+	m := &garmin.MockClient{}
+	svc := garmin.NewSync(m, db, userID, garmin.SyncConfig{MinConfidence: 0.6}, nil)
 
 	today := time.Now()
 	activityIDs := []int64{}