📝 docs(adr): add ADR-125 for usage snapshot SK redesign

[sodre]

Summary

• Proposes redesigning usage snapshot sort key format from #USAGE#{resource}#{window_key} to #USAGE#{window_type}#{resource}#{window_key}
• Introduces rollup snapshots with resource="_all_" for efficient billing dashboard queries
• Enables window type filtering via SK prefix instead of FilterExpression

Motivation

Current design has two limitations:

1. Window type filtering requires FilterExpression (reads all items, filters client-side)
2. Cross-resource time-range queries require reading all snapshots and aggregating client-side

Trade-offs

• Positive: Efficient billing queries, no LSI required, works with existing tables after backfill
• Negative: 2x write amplification in aggregator, ~1.5-2x storage

Test plan

• Review ADR for clarity and completeness
• Validate alternatives considered section captures the brainstorming discussion

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 29.14%. Comparing base (614a777) to head (a29cce7).
⚠️ Report is 382 commits behind head on main.
✅ All tests successful. No failed tests found.

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HEAD has 1 upload less than BASE

Flag	BASE (614a777)	HEAD (a29cce7)
doctest	2	1

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #304      +/-   ##
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- Coverage   37.35%   29.14%   -8.21%     
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  Files          24       37      +13     
  Lines        4645     7854    +3209     
==========================================
+ Hits         1735     2289     +554     
- Misses       2910     5565    +2655     

Flag	Coverage Δ
doctest	29.14% <ø> (-8.21%)	⬇️

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