Add 60 interview-style problems and tracking index

[shiningflash]

Summary

• Adds 60 new interview problems (Problems 6 through 65) across 10 new categories: Fundamentals, SQL Thinking, System Design, Scenarios, Cloud Decisions, Data Modeling, Debugging, Cost and Performance, Streaming, and People and Process.
• Adds PROBLEMS.md — a single index table linking every problem to its question and solution, with category, topic and difficulty tags suitable for filtering on a website.
• Adds Problem 5 (Merging Messy CSVs from Multiple Partners) as a worked example matching the existing Problem 1-4 style.

What each problem contains

Every problem has its own folder with:

• question.md — scenario, task list, and what a good answer should cover.
• solution.md (or solution.py for code-style ones) — a walkthrough written like an experienced engineer would explain it on a whiteboard.

Solutions include diagrams (ASCII / mermaid-style boxes) where the topic benefits from visualization, capacity math where relevant, common-mistake lists, and bonus follow-up questions to anticipate.

Format consistency

All new problems follow the same shape as the existing Problems 1-4:

1. Scenario.
2. Task list.
3. "What a good answer covers."
4. Solution: short version, walkthrough, picture, common mistakes, bonus follow-up.

Coverage

Category	Problems
Fundamentals	6-14
SQL Thinking	15-20
System Design	21-28
Scenarios	29-34
Cloud Decisions	35-40
Data Modeling	41-45
Debugging	46-50
Cost & Performance	51-55
Streaming	56-58
People & Process	59-65

Test plan

• Review PROBLEMS.md table renders correctly on GitHub.
• Spot-check 3-4 problem folders to confirm question/solution pairs are coherent.
• Confirm category and topic labels are useful for filtering on the website.
• Check that links in PROBLEMS.md resolve to the right files (URL-encoded paths).