QA: fix broken links + bilingual blog (/de/blog/)

[a6b8]

A full-site Playwright crawl (all ~116 pages, EN+DE) surfaced broken links and a German-area localization gap.

Findings (real errors)

• /concepts/primitives/ linked to /specification/prompts/ (404) — actual slug is prompt-architecture. (EN+DE)
• German introduction/use-cases.md was never cleaned (only EN was in Memo 064: Webseite Content & UI Audit v2 #87): still had data-source tables, /de/concepts/schema-catalog/ + community-hub links, and bare /de/quickstart/ (404).
• The v4 blog post linked to non-existent /specification/v4.0.0/NN-*.md files (404 ×5).
• German blog area broken: 3 of 4 posts are German but lived in the English /blog/; /de/blog/ did not exist, yet the German homepage Hero linked to it (404).

Note (not a bug)

• The crawl flagged 178 {{...}} "raw template tokens" — these are false positives: FlowMCP's own interpolation syntax shown as documentation in code blocks (spec/guide pages, v4 post). Confirmed intentional.

Fix

• Repoint primitives → /specification/prompt-architecture/; clean German use-cases to mirror EN; fix the v4 post's spec links to real slugs.
• Bilingual blog: lang field per post; German posts under src/content/blog/de/ served at /de/blog/, English translations at /blog/; both index pages + RSS feeds; locale-aware layouts (links, dates, UI strings); EN translations of the 3 German posts + a German welcome.

Verification

• npm run build green (117 pages); re-crawl: 0 broken internal links, 0 console/JS/image/mermaid/title/h1 errors.