📝 docs(adr): add ADR-121 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 37.35%. Comparing base (614a777) to head (3344dd0).
✅ All tests successful. No failed tests found.

Additional details and impacted files

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  Coverage   37.35%   37.35%           
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  Files          24       24           
  Lines        4645     4645           
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  Hits         1735     1735           
  Misses       2910     2910           

Flag	Coverage Δ
doctest	37.35% <ø> (ø)

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