feat(search): index PDF text, OCR scanned PDFs on import

Search now covers handwriting, the PDF text layer, AND scanned
(rasterized) PDFs.

- PdfTextIndexer runs at import: sums the embedded text layer across
  pages; if present it stores that as the document body, otherwise the
  PDF is rasterized and its rendered pages are OCR'd in the background.
  The result lands in the sidecar `pageText` field (distinct from
  `ocrText`, the handwriting OCR). Idempotent (skips a sidecar that
  already has pageText); degrades gracefully with no OCR engine.
- pdfrx_page_text_source abstracts text/render so it's testable.
- VaultSearchIndex now harvests title + typed text + handwriting OCR +
  PDF pageText, so search finds notes, typed PDFs and scanned PDFs.

analyze clean, 409 tests green.

lib/providers/ocr_provider.dart

@@ -1,11 +1,24 @@
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 
 import '../services/ocr_service.dart';
+import '../services/pdf_text_indexer.dart';
+import '../services/pdfrx_page_text_source.dart';
 
 enum OcrStatus { none, processing, done, failed }
 
 final ocrServiceProvider = Provider<OcrService>((ref) => OcrService());
 
+/// The import-time PDF document-body indexer, wired to the pdfrx-backed embedded
+/// text + page-render OCR sources (see [PdfrxPageTextSource]). The import flow
+/// fires [PdfTextIndexer.indexPdf] (fire-and-forget) so a scanned PDF's text
+/// becomes searchable in the background without blocking the editor opening.
+final pdfTextIndexerProvider = Provider<PdfTextIndexer>(
+  (ref) => PdfTextIndexer(
+    loadEmbeddedText: PdfrxPageTextSource.loadEmbeddedText,
+    ocrPages: PdfrxPageTextSource.ocrPages,
+  ),
+);
+
 /// Tracks local OCR processing status per note ID.
 ///
 /// This map only ever holds an entry per note that has had OCR triggered in