fix: use relative delay for Retry-After header on 429

[apoorvdarshan]

Summary

• Fixes Retry-After header on 429 responses to emit a relative delay in seconds (per RFC 9110 §10.2.3) instead of an absolute Unix epoch timestamp
• Previously, Retry-After was set to e.g. 1771404540 (epoch seconds), causing clients that treat it as delay-seconds to wait ~56 years
• X-RateLimit-Reset remains as epoch seconds (standard convention for that header)
• Adds a regression test asserting Retry-After is a small relative value

Closes #407

Test plan

• Existing tests pass (vitest run convex/httpApiV1.handlers.test.ts — 40/40)
• New test "429 Retry-After is a relative delay, not an absolute epoch" verifies the header value is between 1–120 seconds

Greptile Summary

Fixes the Retry-After header on 429 responses to emit a relative delay in seconds (per RFC 9110 §10.2.3) instead of the raw resetAt epoch timestamp. Previously, clients treating the value as delay-seconds would wait ~56 years. The fix computes the delta between the rate-limit reset time and the current time, with a Math.max(1, ...) floor. X-RateLimit-Reset correctly continues to use epoch seconds. A regression test verifies the header value falls within 1–120 seconds.

Confidence Score: 5/5

• This PR is safe to merge — it's a targeted one-line fix with a clear regression test.
• The change is minimal (one new line, one modified line) and directly addresses a clear bug. The relative delay computation is correct: resetAt is an epoch-ms value from the rate limiter, divided by 1000 to get epoch-seconds, then subtracted from the current epoch-seconds. The Math.max(1, ...) guard handles edge cases where the reset window has already passed. The regression test appropriately validates the fix using the existing mock infrastructure.
• No files require special attention.

_{Last reviewed commit: 6c87964}

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Context used:

• Context from dashboard - AGENTS.md (source)

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