Replace flowchart.mmd with per-topic diagrams and a print pipeline

Split the single flowchart.mmd into flowchart/platform-overview.mmd
(all 5 phases) and flowchart/round-lifecycle.mmd (round/draw detail),
plus render-pdf.sh to generate print-ready A4/A3 PDFs with a consistent
theme, header/footer, and legible contrast against the page background.

Also flip the operational policy in CLAUDE.md: the app now always runs
via Docker (dev and prod alike), with the venv reserved for tests,
migration authoring, and one-time secret/key-generation scripts.

.gitignore

@@ -8,3 +8,4 @@ master.xprv.enc
 *.egg-info/
 logs/
 data/
+flowchart/*.pdf