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/cmd/geniusrund/main.go

@@ -45,11 +45,22 @@ func main() {
 	}
 	defer db.Close()
 
-	overrides, err := db.ConfigValues(context.Background())
+	values, err := db.ConfigValues(context.Background())
 	if err != nil {
-		fatal("read app config overrides", err)
+		fatal("read app config", err)
 	}
-	appCfg, err := config.LoadApp(overrides)
+	// Every registry key is mandatory in the DB: seed missing ones with
+	// their defaults so the config table is always fully populated (no
+	// code-side fallbacks anywhere downstream).
+	for _, k := range config.AppRegistry() {
+		if _, ok := values[k.Key]; !ok {
+			if err := db.SetConfigValue(context.Background(), k.Key, k.Default); err != nil {
+				fatal("seed app config default", err)
+			}
+			values[k.Key] = k.Default
+		}
+	}
+	appCfg, err := config.LoadApp(values)
 	if err != nil {
 		fatal("load app config", err)
 	}
@@ -75,15 +86,14 @@ func main() {
 		garmin.SyncConfig{},
 		authVerifier,
 		api.SessionConfig{
-			Secret:      envCfg.SessionSecret,
-			Duration:    appCfg.SessionDuration,
-			Secure:      envCfg.SessionSecure,
-			BackendURL:  envCfg.BackendURL,
-			FrontendURL: envCfg.FrontendURL,
+			Secret:       envCfg.SessionSecret,
+			Duration:     appCfg.SessionDuration,
+			SetupTimeout: appCfg.SetupTimeout,
+			Secure:       envCfg.SessionSecure,
+			BackendURL:   envCfg.BackendURL,
+			FrontendURL:  envCfg.FrontendURL,
 		})
 
-	server.SetupSessionIdleTimeout = appCfg.SetupSessionIdleTimeout
-
 	for _, e := range envCfg.DisplayEnv() {
 		server.EnvVars = append(server.EnvVars, api.EnvVar{Name: e.Name, Value: e.Value})
 	}