fix: Add async/await, PDF metadata, and type safety fixes

This commit addresses multiple issues with async operations, PDF metadata
extraction, and type safety in document processing and search.

## Async/Await Fixes
- processor.py:259 - Added await for chunker.chunk_text(content)
- processor.py:270 - Added await for bm25_service.encode_batch(chunk_texts)
- tests/unit/test_document_chunker.py - Converted all 12 test methods to async

## PDF Metadata Enhancement
- pymupdf.py:143 - Added file_size metadata extraction
- pymupdf.py:145-206 - Refactored to extract text page-by-page
  - Manually loop through pages instead of using page_chunks=True
  - Generate page_boundaries metadata for precise page tracking
  - Works around pymupdf.layout.activate() breaking page_chunks=True
- processor.py:32-66 - Added assign_page_numbers() helper function
  - Assigns page numbers to chunks based on overlap with page boundaries
  - Handles chunks spanning multiple pages
- processor.py:298-300 - Call assign_page_numbers() for PDF files

## Type Safety Fixes
- bm25_hybrid.py:184 - Removed int() conversion of doc_id
- semantic.py:131 - Removed int() conversion of doc_id
- viz_routes.py:275 - Removed int() conversion of doc_id
- Added comments documenting that doc_id can be int (notes) or str (file paths)

## Testing
- All 18 tests passing (12 unit + 6 integration)
- No type errors in modified files
- Container logs show successful processing
- Vector viz searches working correctly

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

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

nextcloud_mcp_server/vector/document_chunker.py

@@ -15,6 +15,8 @@ class ChunkWithPosition:
     text: str
     start_offset: int  # Character position where chunk starts
     end_offset: int  # Character position where chunk ends (exclusive)
+    page_number: int | None = None  # Page number for PDF chunks (optional)
+    metadata: dict | None = None  # Additional processor-specific metadata (optional)
 
 
 class DocumentChunker:
@@ -50,7 +52,7 @@ class DocumentChunker:
             strip_whitespace=True,
         )
 
-    def chunk_text(self, content: str) -> list[ChunkWithPosition]:
+    async def chunk_text(self, content: str) -> list[ChunkWithPosition]:
         """
         Split text into overlapping chunks with position tracking.
 
@@ -66,12 +68,17 @@ class DocumentChunker:
         Returns:
             List of chunks with their character positions in the original content
         """
+        import anyio
+
         # Handle empty content - return single empty chunk for backward compatibility
         if not content:
             return [ChunkWithPosition(text="", start_offset=0, end_offset=0)]
 
-        # Use LangChain to create documents with position tracking
-        docs = self.splitter.create_documents([content])
+        # Run CPU-bound text splitting in thread pool to avoid blocking event loop
+        docs = await anyio.to_thread.run_sync(
+            self.splitter.create_documents,
+            [content],
+        )
 
         # Convert LangChain Documents to ChunkWithPosition objects
         chunks = [