[#916] Fix streak tier drop logic to drop to previous tier

[realproject7]

Summary

• dropOneTier() was snapping to the current tier threshold instead of the previous one
• streak 50 + miss → stayed at 50 (no penalty). Now correctly drops to 30
• streak 100 + miss → stayed at 100. Now correctly drops to 50
• streak 7 + miss → stayed at 7. Now correctly drops to 0
• streak 3 + miss → already returned 0 (unchanged)
• Updated comment in checkin route to match new behavior
• Bumps patch version to 0.1.30

Fixes #916

Test plan

• Verify dropOneTier(100) returns 50
• Verify dropOneTier(50) returns 30
• Verify dropOneTier(7) returns 0
• Verify dropOneTier(3) returns 0
• Check-in flow: miss a day with streak 50, confirm streak drops to 30

🤖 Generated with Claude Code

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1 Skipped Deployment

Project	Deployment	Actions	Updated (UTC)
plotlink	Ignored		Apr 21, 2026 10:01am

[realproject7]

re2 Review — APPROVED ✅

Verified against TIER_THRESHOLDS = [7, 14, 30, 50, 100]:

Input	Old (buggy)	New (fixed)	Expected
100	100	50	50 ✓
50	50	30	30 ✓
30	30	14	14 ✓
7	7	0	0 ✓
3	0	0	0 ✓

The fix is a single index shift: TIER_THRESHOLDS[i] → TIER_THRESHOLDS[i - 1] with a bounds check. Comment in checkin route updated to match. Patch bump 0.1.30 ✓

[project7-interns]

Verdict: APPROVE

Summary

This PR corrects dropOneTier() to drop to the previous streak threshold instead of snapping to the current one. The implementation now matches issue #916's stated behavior for 100, 50, 7, and sub-7 streaks.

Findings

• None.

Decision

Approve. I verified the threshold ordering and the updated logic now yields the intended tier drops. CI checks were still pending at review time.