feat: export analysis reports as PDF (closes #138)

[luceinaltis]

Summary

Adds a PDF export option to ReportExporter so analysis results can be shared as formatted PDFs alongside the existing Markdown and JSON formats. This is a small, focused re-implementation targeting current main after #144 was closed for being branched from the pre-refactor src/tracer layout.

Closes #138.

What changed

• qracer/conversation/report_exporter.py — new save_pdf() method. Renders header, query, response body, optional Trade Thesis, and successful data sources via fpdf2. The import is lazy so ReportExporter keeps working without the optional dep; callers get a clear ImportError with install instructions instead of a hard crash at module import.
• qracer/conversation/engine.py — save_last_report(fmt="pdf") routes to the new method.
• qracer/cli.py — save pdf / /save pdf REPL commands, with a friendly message when fpdf2 is missing. Help text and banner updated.
• pyproject.toml — new [pdf] = ["fpdf2>=2.7.0"] optional extra, plus fpdf2 added to the dev dependency group so CI exercises the feature end-to-end (CI runs uv sync --extra web, which pulls in the dev group).
• tests/conversation/test_report_exporter.py — 4 new tests covering basic write, Trade Thesis rendering, data-sources filtering (only successful tools), and the general-ticker fallback. Because fpdf2 zlib-compresses page streams, a small _extract_pdf_text helper decompresses FlateDecode blocks so assertions can look for literal text without pulling in a PDF parser. Each PDF test uses pytest.importorskip("fpdf") to stay opt-in on environments without the extra.

How to test

uv sync --extra web --extra pdf
uv run ruff check .
uv run ruff format --check .
uv run pyright
uv run pytest tests/conversation/test_report_exporter.py -v

Or exercise the new REPL command manually:

uv run qracer repl
> Analyze AAPL
> save pdf
# -> Saved to ~/.qracer/reports/AAPL-YYYY-MM-DD.pdf

The generated PDF contains the header, query, response, Trade Thesis (when present), and the list of successful data sources.

Notes

• Full suite (pytest --cov=qracer --cov-fail-under=80) passes locally at 80.37% coverage with all 711 tests green.
• ruff check, ruff format --check, and pyright all clean.
• save_pdf() intentionally mirrors save_markdown()'s "successful tools only" behavior for the Data Sources section so the three formats stay consistent.