fix(api): guard Upstash rate-limit failures on non-keeper routes

[QwinDivy]

close #639

Summary

• checkRateLimit's call to Upstash (ratelimit.limit(ip)) was unguarded on 6 routes (tx/deposit, tx/withdraw, tx/submit, tx/add-trustline, positions/[publicKey], vaults/index, vaults/[vaultId]), unlike the keeper routes which already catch this failure and return a clean 503.
• All 6 now wrap the call the same way, logging a route-specific message and returning 503 {"error": "Rate limiter unavailable; refusing to run"} instead of crashing unguarded on a transient Upstash timeout.

Test plan

• pnpm lint && pnpm typecheck pass locally
• pnpm test (api package): 46/46 pass, including the new regression test asserting a 503 on a rejected checkRateLimit

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[collinsezedike]

The fix itself is correct: all 6 remaining routes get the same try/catch → log → 503 pattern already used in the keeper routes, and the new regression test covers it.

CI fails on three checks though:

• PR Title: "Guard Upstash rate-limit failures on non-keeper routes" doesn't follow the conventional-commit format (type(scope): subject) this repo requires. Please rename to something like fix(api): guard Upstash rate-limit failures on non-keeper routes.
• Commit Messages: same root cause, the commit header has no type prefix (type may not be empty, subject may not be empty).
• Lint & Typecheck: prettier --check fails on api/__tests__/handlers.test.ts. Run pnpm format and push.

The PR body's Summary and Test plan sections are also still empty (the raw template). Please fill those in per CONTRIBUTING.md before this is ready for merge.

One thing worth considering, not blocking: the try/catch/503 block is now duplicated verbatim across 8 call sites. Might be worth pushing that into checkRateLimit itself so a future change to the guard doesn't need to be copy-edited into 8 files by hand, but that's a design call, not something I'd hold up this fix for.

[collinsezedike]

Merging now.