fix: reduce itinerary SVG font payload

[Minorli]

Summary

• subset the embedded itinerary title font to only glyphs used by the current title and subtitle
• preserve the existing self-contained SVG font and visual styling while reducing client decode pressure
• fall back to system fonts if subsetting fails and cap the glyph-set cache
• release PicGen 0.1.63

Production finding

A deployed 0.1.62 customer-style run produced a 12.7 MB itinerary SVG and Chromium crashed once while loading/compositing it. A controlled identical-background comparison reduced the SVG from 10,093,980 bytes to 1,689,480 bytes (83% reduction).

Test plan

• ./scripts/check.sh (408 passed; Ruff and Mypy clean)
• uv run --with pytest-cov pytest -q --cov=picgen --cov-report=term (87% total)
• uv run --with pip-audit pip-audit (no known vulnerabilities)
• old/new Chrome render comparison at 1792x1792 (0.042% tiny antialiasing-only pixel delta)
• 20 repeated SVG decode + logo canvas composition + PNG encode cycles
• Docker build and non-root healthy-container smoke test