refactor(config): mandatory app-config rows seeded in DB; rename to session.setup_timeout

All registry keys must exist as rows in the config table: main seeds
missing keys with their defaults at startup, LoadApp fails fast on a
missing key, and the code-side fallback const/helper for the onboarding
setup timeout is gone -- the value rides in SessionConfig.SetupTimeout.
The key is renamed session.idle_timeout -> session.setup_timeout, and
the /config page's 'overridden' now means 'differs from the default'.

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

backend/internal/api/server.go

@@ -53,11 +53,6 @@ type Server struct {
 	// never call os.Getenv.
 	EnvVars []EnvVar
 
-	// SetupSessionIdleTimeout is the app-config session.idle_timeout value
-	// (see internal/config); zero falls back to
-	// defaultSetupSessionIdleTimeout.
-	SetupSessionIdleTimeout time.Duration
-
 	mu              sync.Mutex
 	userClient      map[int64]garmin.Client
 	userSync        map[int64]*garmin.Sync