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/handlers/admin.go

@@ -574,6 +574,7 @@ func (a *App) AdminSettingsPost(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	settings.AnonymousUploadsEnabled = r.FormValue("anonymous_uploads_enabled") == "on"
+	settings.ResumableUploadsEnabled = r.FormValue("resumable_uploads_enabled") == "on"
 	if value := parsePositiveInt(r.FormValue("usage_retention_days")); value > 0 {
 		settings.UsageRetentionDays = value
 	}
@@ -604,6 +605,16 @@ func (a *App) AdminSettingsPost(w http.ResponseWriter, r *http.Request) {
 	if value := parsePositiveInt(r.FormValue("short_window_seconds")); value > 0 {
 		settings.ShortWindowSeconds = value
 	}
+	if value := parsePositiveFloat(r.FormValue("resumable_chunk_size_mb")); value > 0 {
+		settings.ResumableChunkSizeMB = value
+	}
+	if value := parsePositiveInt(r.FormValue("resumable_retention_hours")); value > 0 {
+		settings.ResumableRetentionHours = value
+	}
+	if value := strings.TrimSpace(r.FormValue("resumable_chunk_mode")); value != "" {
+		settings.ResumableChunkMode = value
+	}
+	settings.ResumableChunkPath = strings.TrimSpace(r.FormValue("resumable_chunk_path"))
 	if value := r.FormValue("anonymous_storage_backend"); value != "" {
 		settings.AnonymousStorageBackend = value
 	}