feat: add storage content + 61 Q&As to 07-databases.html

Added "Storage in System Design" course content covering structured vs
unstructured data, 4 storage categories (Database/Object/File/Block),
storage properties, CAP Theorem examples, and trade-off patterns.

Added 61 interview Q&As across 6 sub-groups: Introduction to Storage,
Database Performance Optimization, Advanced Database topics, File Systems
& Distributed Storage, Object Storage, and Big Data.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: sridhar-3009