feat(config): allow -1 to represent unlimited upload limits

Introduce support for configuring unlimited upload limits by allowing -1
as a valid value for anonymous and user upload MB limits.

Changes include:
- Added `envMegabytesLimitFloat` and helper functions to parse and validate limits where -1 is allowed.
- Updated validation logic to accept -1 for `AnonymousMaxUploadMB`, `AnonymousDailyUploadMB`, and `UserDailyUploadMB`.
- Added a test case to verify unlimited upload policy behavior.

backend/libs/config/config.go

@@ -72,9 +72,9 @@ func Load() (Config, error) {
 		MaxUploadSize:    envMegabytes("WARPBOX_MAX_UPLOAD_SIZE_MB", 2048), // 2 GiB default.
 		DefaultSettings: SettingsDefaults{
 			AnonymousUploadsEnabled: envBool("WARPBOX_ANONYMOUS_UPLOADS_ENABLED", true),
-			AnonymousMaxUploadMB:    envMegabytesFloat("WARPBOX_ANONYMOUS_MAX_UPLOAD_MB", 512),
-			AnonymousDailyUploadMB:  envMegabytesFloat("WARPBOX_ANONYMOUS_DAILY_UPLOAD_MB", 2048),
-			UserDailyUploadMB:       envMegabytesFloat("WARPBOX_USER_DAILY_UPLOAD_MB", 8192),
+			AnonymousMaxUploadMB:    envMegabytesLimitFloat("WARPBOX_ANONYMOUS_MAX_UPLOAD_MB", 512),
+			AnonymousDailyUploadMB:  envMegabytesLimitFloat("WARPBOX_ANONYMOUS_DAILY_UPLOAD_MB", 2048),
+			UserDailyUploadMB:       envMegabytesLimitFloat("WARPBOX_USER_DAILY_UPLOAD_MB", 8192),
 			DefaultUserStorageMB:    envMegabytesFloat("WARPBOX_DEFAULT_USER_STORAGE_MB", 51200),
 			UsageRetentionDays:      envInt("WARPBOX_USAGE_RETENTION_DAYS", 30),
 			LocalStorageMaxGB:       envGigabytesFloat("WARPBOX_LOCAL_STORAGE_MAX_GB", 100),
@@ -97,9 +97,9 @@ func Load() (Config, error) {
 	if cfg.MaxUploadSize <= 0 {
 		return Config{}, fmt.Errorf("WARPBOX_MAX_UPLOAD_SIZE_MB must be positive")
 	}
-	if cfg.DefaultSettings.AnonymousMaxUploadMB <= 0 ||
-		cfg.DefaultSettings.AnonymousDailyUploadMB <= 0 ||
-		cfg.DefaultSettings.UserDailyUploadMB <= 0 ||
+	if !validUnlimitedMegabyteLimit(cfg.DefaultSettings.AnonymousMaxUploadMB) ||
+		!validUnlimitedMegabyteLimit(cfg.DefaultSettings.AnonymousDailyUploadMB) ||
+		!validUnlimitedMegabyteLimit(cfg.DefaultSettings.UserDailyUploadMB) ||
 		cfg.DefaultSettings.DefaultUserStorageMB <= 0 ||
 		cfg.DefaultSettings.UsageRetentionDays <= 0 ||
 		cfg.DefaultSettings.LocalStorageMaxGB <= 0 ||
@@ -111,7 +111,7 @@ func Load() (Config, error) {
 		cfg.DefaultSettings.UserActiveBoxes <= 0 ||
 		cfg.DefaultSettings.ShortWindowRequests <= 0 ||
 		cfg.DefaultSettings.ShortWindowSeconds <= 0 {
-		return Config{}, fmt.Errorf("upload policy settings must be positive")
+		return Config{}, fmt.Errorf("upload policy settings must be positive, with -1 allowed for upload MB limits")
 	}
 
 	return cfg, nil
@@ -203,6 +203,18 @@ func envMegabytesFloat(key string, fallback float64) float64 {
 	return parsed
 }
 
+func envMegabytesLimitFloat(key string, fallback float64) float64 {
+	value := strings.TrimSpace(os.Getenv(key))
+	if value == "" {
+		return fallback
+	}
+	parsed, err := parseMegabytesLimitFloat(value)
+	if err != nil {
+		return fallback
+	}
+	return parsed
+}
+
 func envGigabytesFloat(key string, fallback float64) float64 {
 	value := strings.TrimSpace(os.Getenv(key))
 	if value == "" {
@@ -246,6 +258,35 @@ func parseMegabytesFloat(value string) (float64, error) {
 	return sizeMB, nil
 }
 
+func parseMegabytesLimitFloat(value string) (float64, error) {
+	sizeMB, err := parseMegabytesFloatAllowNegativeOne(value)
+	if err != nil {
+		return 0, err
+	}
+	if !validUnlimitedMegabyteLimit(sizeMB) {
+		return 0, fmt.Errorf("megabyte value must be positive or -1 for unlimited")
+	}
+	return sizeMB, nil
+}
+
+func parseMegabytesFloatAllowNegativeOne(value string) (float64, error) {
+	normalized := strings.TrimSpace(value)
+	normalized = strings.TrimSuffix(normalized, "MB")
+	normalized = strings.TrimSuffix(normalized, "Mb")
+	normalized = strings.TrimSuffix(normalized, "mb")
+	normalized = strings.TrimSpace(normalized)
+
+	sizeMB, err := strconv.ParseFloat(normalized, 64)
+	if err != nil {
+		return 0, fmt.Errorf("invalid megabyte value %q: %w", value, err)
+	}
+	return sizeMB, nil
+}
+
+func validUnlimitedMegabyteLimit(value float64) bool {
+	return value > 0 || value == -1
+}
+
 func megabytesToBytes(sizeMB float64) int64 {
 	return int64(math.Round(sizeMB * 1024 * 1024))
 }