PRD preview, PDF export, syntax highlighting, security hardening

[TomMaSS]

Summary

• PRD Preview Dialog — in-app modal rendering full PRD markdown with syntax highlighting, not raw text in a new tab
• Export buttons — .md download + .pdf via browser print (clean light-theme layout), available both in tab bar and preview dialog
• Syntax highlighting — highlight.js github-dark theme for code blocks in section viewer and preview
• Security hardening — member endpoint auth, ws-token derived from session, schema-qualified to_regclass, default secret warnings
• Schema fix — auth user ID columns UUID → TEXT (Better Auth uses random strings)
• Demo GIF — Playwright script rewritten, 24s walkthrough regenerated
• GitHub repo metadata — description + 10 topic tags
• install.sh — service count check bumped for redis

Test plan

• pytest tests/ -x -v — 171 tests pass
• npx tsc --noEmit — typecheck clean
• Preview button opens modal with rendered markdown
• .md button downloads markdown file
• .pdf button opens print dialog (works from tab bar without preview)
• Code blocks have syntax highlighting (SQL, Python, YAML)
• install.sh --help works, syntax valid
• All 5 services healthy after rebuild