Swap PDF reports to DejaVu Sans (fix Unicode black squares)

[eagnespuerto]

Both the transit report (report.py) and the microlensing report (microlensing_report.py) were rendering Greek letters, superscripts, and astronomy symbols (θ, Δ, μ, ², °, ⊕, ±, ≈, ≥, …) as black squares because Helvetica's built-in WinAnsi subset doesn't cover them. This landed hardest on the microlensing PDF, whose observables + planet-predictions tables use θ_E, μ_rel, M⊕/M♃, ΔBIC, and per-parameter ± error notation on almost every row.

Fix: register DejaVu Sans (regular + bold) as reportlab TrueType fonts on module import, and swap the hardcoded "Helvetica" / "Helvetica-Bold" strings for the resolved font names. DejaVu ships bundled with matplotlib (already a Vetstar dep) so no new install is needed and no system-font path assumption creeps in — production containers on Render and Fly.io will have it automatically.

Changes

• New backend/app/pdf_fonts.py: ensure_dejavu_registered() + font_normal() / font_bold(). Idempotent. Looks up matplotlib.get_data_path()/fonts/ttf/DejaVuSans{,-Bold}.ttf, registers via reportlab.pdfbase.pdfmetrics, and binds a font family so Paragraph inline ... markup uses the bold face. Silent no-op if matplotlib/DejaVu aren't available (falls back to Helvetica with a warning — same behaviour as before, no regression).
• backend/app/report.py: import + apply FONT_NORMAL / FONT_BOLD across every TableStyle "FONTNAME" cmd, canvas.setFont, and _build_styles() ParagraphStyle (which previously inherited Helvetica from getSampleStyleSheet's Heading1/BodyText — that inheritance is now explicitly overridden per style).
• backend/app/microlensing_report.py: same swap, plus ml_small ParagraphStyle now sets fontName explicitly (was inheriting from Normal → Helvetica).

Verified via pypdf text extraction: Greek + symbol glyphs that appear in the microlensing PDF (θ, Δ, σ, °, ⊕, ±, ≈, μ, ≥) now round-trip correctly through the DejaVuSans font stream — no black squares. PDF font resource dictionary shows /AAAAAA+DejaVuSans and /AAAAAA+DejaVuSans-Bold (subsetted embeddings) alongside the inescapable base14 /Helvetica reportlab always writes.

Tests: 45/45 microlensing tests still pass.