📝 docs(infra): add multi-region architecture ADRs (123-129)

[sodre]

Summary

• Add seven proposed ADRs documenting the multi-region rate limiting architecture
• ADR-123: Independent regional tables (reject Global Tables)
• ADR-124: S3-based cross-region sync exchange
• ADR-125: Quota enforcement via entity config overrides
• ADR-126: Trigger-based sync writes (exhaustion + drift)
• ADR-127: Per-region sync Lambda (symmetric, no coordinator)
• ADR-128: TTL on sync-written config records (extends ADR-119)
• ADR-129: Sync config ownership via TTL presence

Test plan

• Review each ADR for technical accuracy and consistency
• Verify ADR numbering does not conflict with existing ADRs
• Confirm all ADRs have "Proposed" status

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 29.41%. Comparing base (0858e59) to head (e4b7a8b).
⚠️ Report is 5 commits behind head on main.
✅ All tests successful. No failed tests found.

❗ There is a different number of reports uploaded between BASE (0858e59) and HEAD (e4b7a8b). Click for more details.

HEAD has 3 uploads less than BASE

Flag	BASE (0858e59)	HEAD (e4b7a8b)
unit	1	0
e2e	1	0
integration	1	0

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #393       +/-   ##
===========================================
- Coverage   91.87%   29.41%   -62.47%     
===========================================
  Files          33       33               
  Lines        7435     7435               
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- Hits         6831     2187     -4644     
- Misses        604     5248     +4644     

Flag	Coverage Δ
doctest	29.41% <ø> (+1.42%)	⬆️
e2e	?
integration	?
unit	?

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