fix: Centralize PDF processing and generate separate images per chunk

Previously, pymupdf4llm.to_markdown() was called twice - once in
PyMuPDFProcessor during indexing and again in PDFHighlighter during
visualization. Different image path lengths caused different character
offsets, leading to highlighted pages not matching their chunks.

Also fixed issue where all chunks on the same page showed all highlights
instead of just their own highlight. Now restores original page contents
between chunks using xref stream caching.

Changes:
- Add PDFHighlighter class requiring pre-computed page_boundaries and
  full_text from document processor (no fallback extraction)
- Pass pre-computed data from processor to highlighter
- Extract page-relative portion of chunk text for cross-page chunks
- Add bounding box highlighting using text anchor search
- Run highlight generation in parallel with embedding/BM25
- Cache and restore page contents to isolate highlights per chunk

Results: Highlighting success rate improved from 51% to 95% (121/128).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

nextcloud_mcp_server/document_processors/pymupdf.py

@@ -102,7 +102,7 @@ class PyMuPDFProcessor(DocumentProcessor):
                 await progress_callback(0, 100, "Processing PDF in background thread")
 
             # Run CPU-bound PDF processing in thread pool to avoid blocking event loop
-            result = await anyio.to_thread.run_sync(
+            result = await anyio.to_thread.run_sync(  # type: ignore[attr-defined]
                 self._process_sync,
                 content,
                 filename,
@@ -240,7 +240,7 @@ class PyMuPDFProcessor(DocumentProcessor):
         # Basic document info
         metadata["page_count"] = doc.page_count
         metadata["format"] = "PDF 1." + str(
-            doc.pdf_version() if hasattr(doc, "pdf_version") else "?"
+            doc.pdf_version() if hasattr(doc, "pdf_version") else "?"  # type: ignore[call-non-callable]
         )
 
         if filename: