feat(spreads): expose spreadPct alongside spreadBps for easier display

[sadiquhammedabubakar-art]

Overview

Adds a spreadPct field to each venue in the /spreads/:asset response, alongside the existing spreadBps, so clients can render a percentage without converting.

Closes #93

Changes

• src/routes/spreads.ts: add spreadPct: number to the VenueSpread interface and compute it as ((ask - bid) / mid) * 100 — the same mid-referenced formula as spreadBps, just scaled to percent.
• src/__tests__/spreads.test.ts: asserts spreadPct === spreadBps / 100 and that bid 99 / ask 101 yields 2%.

Verification

• npx tsc --noEmit — clean
• npx vitest run src/__tests__/spreads.test.ts — 12/12 passed
• full suite: 398 passed / 402, the 3 failures are the pre-existing flaky env-isolation timeouts tracked in Flaky test suite: auth.test.ts and pairs.test.ts fail intermittently (~1 in 5 runs) #151

(Note: the original description on this PR was pasted from an unrelated project and has been replaced.)

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