Webhook retries have no jitter, so failures synchronise #341

Background

src/alerts/delivery.ts backs off deterministically:

function retryDelayMs(attempt: number): number {
  return Math.pow(2, attempt - 1) * 1000
}

Every delivery that fails at the same moment retries at exactly the same moment
— 1s, then 2s, then 4s. A price movement is precisely the event that triggers
many alerts at once, so when a subscriber's endpoint has a wobble, Lens responds
by hitting it with a synchronised burst, three times, which is a good way to
turn a wobble into an outage.

What to build

• Add jitter. Full jitter — a random value between zero and the computed delay —
  is the standard choice and is one line.
• Keep the existing retry count and the existing rule that 4xx is not retried,
  which is correct.
• Make the test deterministic by injecting the randomness rather than asserting
  on a range.

Acceptance criteria

• Retry delays are jittered
• Two deliveries failing simultaneously retry at different times, asserted by a test
• 4xx responses are still not retried
• The attempt cap is unchanged
• The randomness source is injectable so the test is not flaky

Drips Wave · Complexity: Easy · 100 points

Repo: Divinity-Labs/Lens. Node + TypeScript, Fastify, Prisma on Postgres,
Redis for queues, Prometheus via prom-client.

Run the tests: npm test. The tests are vitest, not jest. Running jest
against this repo reports "66 suites, 0 tests" and means nothing. A single file
is npx vitest run src/__tests__/yourfile.test.ts.

Load test (prices) is already failing on main and has been for some
time. It is not your PR that broke it. Every other check should be green.

This service prices real assets on Stellar mainnet. A number that is wrong
is worse than a number that is missing, because a caller cannot tell the
difference. Prefer failing loudly over returning a plausible value.

Never elide an address, a contract id or a key in code, a comment or a test
fixture. A previous batch wrote a shortened contract id in an issue text and
five contributors invented plausible-looking ids to fill the gap. Write the
whole thing or do not write it at all.

The house pattern for an outbound deadline already exists — see
src/alerts/delivery.ts, which passes signal: AbortSignal.timeout(10_000) to
fetch. Where an issue below asks for a timeout, copy that, do not invent a
second convention.

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