docs(module21): add day 5 production grade plan

[senih25]

Purpose

Complete Kaggle Module 21 Day 5 as a repo-safe production-readiness mapping.

Scope

• Adds Day 5 spec-driven production-grade development plan.
• Keeps optional Cloud Run and frontend codelabs gated behind manual review.
• Includes a small Day 4 type-hint alignment for the existing security guardrail flow.

Verification

• uv run pytest tests/unit/test_security_guardrails.py -q -> 4 passed
• git diff --check -> clean

Safety

• No PHI
• No secrets
• No .env
• No cloud deploy
• No API enablement
• No Secret Manager changes
• No public endpoint