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/internal/config/appconfig.go

@@ -63,7 +63,7 @@ type AppConfig struct {
 // LoadApp merges overrides (store.ConfigValues' map) over defaults. Pure
 // -- it takes the raw map instead of a *store.DB so this package needs no
 // store dependency and the merge logic tests without a database. A bad
-// stored value fails fast, same posture as Load() for env vars.
+// stored value fails fast, same posture as LoadEnv() for env vars.
 func LoadApp(overrides map[string]string) (AppConfig, error) {
 	merged := map[string]string{}
 	for _, k := range appRegistry {