feat(upload): add resumable chunk configuration and file validation

- Add `WARPBOX_RESUMABLE_CHUNK_MODE` and `WARPBOX_RESUMABLE_CHUNK_PATH` environment variables to configure temporary chunk storage.
- Implement strict file validation for resuming uploads to ensure selected files match the pending session's metadata.
- Add `PLANS.md` to document development stages, roadmap, and API specifications (including batching and resumable flows).

backend/libs/config/config.go

@@ -55,6 +55,11 @@ type SettingsDefaults struct {
 	ShortWindowSeconds      int
 	AnonymousStorageBackend string
 	UserStorageBackend      string
+	ResumableUploadsEnabled bool
+	ResumableChunkSizeMB    float64
+	ResumableRetentionHours int
+	ResumableChunkMode      string
+	ResumableChunkPath      string
 }
 
 func Load() (Config, error) {
@@ -100,8 +105,13 @@ func Load() (Config, error) {
 			ShortWindowSeconds:      envInt("WARPBOX_SHORT_WINDOW_SECONDS", 60),
 			AnonymousStorageBackend: envString("WARPBOX_ANONYMOUS_STORAGE_BACKEND", "local"),
 			UserStorageBackend:      envString("WARPBOX_USER_STORAGE_BACKEND", "local"),
+			ResumableChunkMode:      envString("WARPBOX_RESUMABLE_CHUNK_MODE", "same"),
+			ResumableChunkPath:      envString("WARPBOX_RESUMABLE_CHUNK_PATH", ""),
 		},
 	}
+	cfg.DefaultSettings.ResumableUploadsEnabled = cfg.ResumableUploadsEnabled
+	cfg.DefaultSettings.ResumableChunkSizeMB = float64(cfg.ResumableChunkSize) / 1024 / 1024
+	cfg.DefaultSettings.ResumableRetentionHours = int(cfg.ResumableRetention / time.Hour)
 
 	if cfg.BaseURL == "" {
 		return Config{}, fmt.Errorf("WARPBOX_BASE_URL cannot be empty")