Add Section Headers to Lecture: What Is Normalization in SQL?

Part of #69286

Summary

The lecture challenge "What Is Normalization in SQL?" is ~1285 words with no section headers, making it hard to scan.

• File: curriculum/challenges/english/blocks/lecture-working-with-sql/68816f386bc30d36f59e9563.md
• Word count: ~1285

Acceptance Criteria

• Add ## section headers to break the lecture body (the # --description-- section) into logical sections
• Headers should reflect the natural structure of the content; no rewriting of existing prose
• See curriculum/challenges/english/blocks/lecture-working-with-higher-order-functions-and-callbacks/673362d7f94d551edb532d24.md ("How Does the Sort Method Work?") for the existing convention: short, Title Case ## headers placed before each major sub-topic
• Do not modify the # --questions-- section, code blocks, or front matter
• Ensure the lecture still reads clearly end-to-end after headers are added

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