docs: autonomous architectural standardization for fastify

[beginwebdev2002]

• Replaced non-inclusive language ('master' -> 'primary') in Redis and MongoDB backend documentation.
• Created backend/fastify/readme.md applying the strict 4-step lifecycle (Bad Practice, Problem, Best Practice, Solution), including Markdown and Mermaid formatting constraints as per the knowledge base rules.
• Linked Fastify documentation in backend/readme.md strictly in alphabetical order.
• Regenerated the sitemap and validated compliance via generate-sitemap.js, vibe-check-runner.js, and commit_verify.py.

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PR created automatically by Jules for task 15414411063885591710 started by @beginwebdev2002

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