Add Paperless-ngx PDF sync provider with document versioning and task polling

[Copilot]

Paperless-ngx's POST /api/documents/post_document/ always creates a new document, so repeated syncs of a modified app document would accumulate duplicates. The upload endpoint also returns only a task UUID — the actual Paperless document ID is only available once the background task completes.

API wrapper (PaperlessNgx.ts)

• fetchTasks(service) — GET /api/tasks/, returns full task list with task_id, status, related_document
• updateDocumentVersion(service, paperlessDocId, title, fileData) — POST /api/documents/{id}/update_version/ for subsequent syncs
• PaperlessTask / PaperlessTaskStatus types covering all Celery states (PENDING, STARTED, SUCCESS, FAILURE, REVOKED, RETRY, RECEIVED)
• Minor fixes: getAuthHeaders accepts string | undefined; endpoint defaults to '' in makeRequest

Sync service (PaperlessNgxPDFSyncService.ts)

• Stores the linked Paperless document ID in doc.extra['paperless_pdf_id'] (persisted to DB)
• On writePDF:
  • Existing link → updateDocumentVersion (no duplicate created)
  • New document → uploadDocument → register task UUID → await polling resolution → persist ID
• Polling loop (pollLoop) runs every 2 s, shared across concurrent uploads for the same service instance; each task UUID is tracked in pendingTasks: Map<uuid, {resolve, reject}> so writePDF awaits only its own task

// First sync: uploads and waits for Paperless to finish consuming
const taskUuid = await uploadDocument(this, fileName, file);
const paperlessDocId = await this.waitForTask(taskUuid);   // polls /api/tasks/ every 2s
await document.save({ extra: { paperless_pdf_id: paperlessDocId } }, false, false);

// Subsequent syncs: updates the existing document in-place
await updateDocumentVersion(this, existingPaperlessId, fileName, file);