fix: add legacy /app/config.json fallback for container upgrades

internal/config/paths.go

@@ -38,6 +38,14 @@ func ConfigPath() string {
 	return ResolvePath("DS2API_CONFIG_PATH", "config.json")
 }
 
+func legacyContainerConfigPath() string {
+	return "/app/config.json"
+}
+
+func shouldTryLegacyContainerConfigPath() bool {
+	return strings.TrimSpace(os.Getenv("DS2API_CONFIG_PATH")) == "" && BaseDir() == "/app"
+}
+
 func RawStreamSampleRoot() string {
 	return ResolvePath("DS2API_RAW_STREAM_SAMPLE_ROOT", "tests/raw_stream_samples")
 }

internal/config/store.go

@@ -87,12 +87,17 @@ func loadConfig() (Config, bool, error) {
 		}
 		return cfg, true, err
 	}
-
 	cfg, err := loadConfigFromFile(ConfigPath())
 	if err != nil {
+		if shouldTryLegacyContainerConfigPath() {
+			legacyPath := legacyContainerConfigPath()
+			if legacyCfg, legacyErr := loadConfigFromFile(legacyPath); legacyErr == nil {
+				Logger.Info("[config] loaded legacy container config path", "path", legacyPath)
+				return legacyCfg, false, nil
+			}
+		}
 		if IsVercel() {
-			// Vercel one-click deploy may start without a writable/present config file.
-			// Keep an in-memory config so users can bootstrap via WebUI then sync env.
+			// Vercel may start without writable/present config; keep in-memory bootstrap config.
 			return Config{}, true, nil
 		}
 		return Config{}, false, err