Add PDF export functionality using WeasyPrint

- Introduced `PdfExporter` class for exporting meeting transcripts to PDF format.
- Updated `ExportService` to support PDF as a new export format.
- Enhanced `proto_to_export_format` to handle PDF format.
- Added WeasyPrint as a dependency in `pyproject.toml` and `uv.lock`.
- Updated documentation to reflect the new PDF export capabilities and resolved issues from Sprint 3.
- Implemented tests for PDF export functionality, ensuring valid PDF output and proper handling of meeting data.

pyproject.toml

@@ -30,6 +30,7 @@ dependencies = [
     "diart>=0.9.2",
     # HTTP client for webhooks and integrations
     "httpx>=0.27",
+    "weasyprint>=67.0",
 ]
 
 [project.optional-dependencies]