feat(week-10): add dbt SQL practice exercises

[lassebenni]

Summary

• Adds data-track/week-10/ with 4 dbt SQL exercises for Week 10 (dbt Core)
• Exercises are dbt model/macro/test files students copy into their existing project (same pattern as Week 9's SQL files, adapted for dbt's file types)
• Each exercise has a starter file with TODO comments, a README with success criteria, and a solutions/ folder with WHY-annotated reference implementations

Exercises

Exercise	Skill	File(s)
1	safe_divide macro + computed columns in stg_trips	macros/safe_divide.sql, models/staging/stg_trips.sql
2	Singular test (FAIL 182 → PASS cycle)	tests/assert_fare_amount_non_negative.sql
3	Debug a broken ref() at compile time	models/marts/fct_trips.sql (broken version)
4	Propagate a column change from staging to mart	models/staging/stg_trips.sql, models/marts/fct_trips.sql

Test plan

• All starter files contain TODO comments, no pre-satisfied answers
• All solution files contain -- WHY annotation on non-obvious choices
• Exercise 2 instructs students to first run the test against unfiltered data (FAIL 182) before restoring the filter (PASS), proving the test actually catches the issue
• Exercise 3 includes only the broken file; the typo is in a {{ ref() }} call so dbt's compile-time error points directly at it
• Exercise 4 READMEs explain why +fct_trips is needed vs plain fct_trips

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