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/templates/pages/admin_settings.html

@@ -126,6 +126,33 @@
             </label>
           </div>
 
+          <div class="settings-section">
+            <h3 class="settings-section-title">Resumable uploads</h3>
+            <label class="checkbox-field">
+              <input type="checkbox" name="resumable_uploads_enabled" {{if .Data.Settings.ResumableUploadsEnabled}}checked{{end}}>
+              <span>Enable browser/API resumable uploads</span>
+            </label>
+            <label>
+              <span>Chunk size (MB)</span>
+              <input name="resumable_chunk_size_mb" value="{{.Data.Settings.ResumableChunkSizeMB}}" required>
+            </label>
+            <label>
+              <span>Draft retention (hours)</span>
+              <input type="number" name="resumable_retention_hours" min="1" value="{{.Data.Settings.ResumableRetentionHours}}" required>
+            </label>
+            <label>
+              <span>Chunk storage</span>
+              <select name="resumable_chunk_mode" required>
+                <option value="same" {{if eq .Data.Settings.ResumableChunkMode "same"}}selected{{end}}>Default local data path</option>
+                <option value="custom" {{if eq .Data.Settings.ResumableChunkMode "custom"}}selected{{end}}>Custom local path, e.g. fast SSD</option>
+              </select>
+            </label>
+            <label>
+              <span>Custom chunk path</span>
+              <input name="resumable_chunk_path" value="{{.Data.Settings.ResumableChunkPath}}" placeholder="/mnt/fast-ssd/warpbox-chunks">
+            </label>
+          </div>
+
           <button class="button button-primary" type="submit">Save settings</button>
         </form>
       </div>