Add background thread PDF discovery with progress dialog and archiving

- Add PDFDiscoveryWorker QThread for non-blocking PDF downloads
- Add progress dialog with real-time status updates during discovery
- Implement year-based PDF archiving (same as full BMLibrarian)
- Check for existing local PDFs before attempting download
- Use PDF_BASE_DIR env var or ~/knowledgebase/pdf default
- Update load_from_citation to use async discovery
- Update _do_pdf_discovery (Fetch PDF button) to use async discovery
- Add helper methods: _get_pdf_base_dir, _generate_pdf_path, _find_existing_pdf

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: hherb