redis-cache.ts's webhook-dedup path records no error metric, unlike its two cache siblings

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Context

src/selfhost/redis-cache.ts implements webhook-delivery-dedup helpers isWebhookDeliveryDuplicate
(lines 18-29) and rememberWebhookDelivery (lines 32-38). Both wrap their Redis calls in a catch
block that silently swallows errors — no error metric is recorded on failure.

Both of this file's Redis-cache siblings record an error metric on the exact same failure mode:

• src/selfhost/redis-token-cache.ts calls recordTokenCacheMetric("error") in both get() and
  set()'s catch blocks.
• src/selfhost/redis-response-cache.ts calls recordRedisResponseCacheMetric("error") in both
  get() and set()'s catch blocks.

redis-cache.ts already imports incr and uses it for the dedup-hit case, but has no equivalent
metric call for the error case. As a result, a sustained Redis outage affecting webhook dedup is
completely invisible in metrics/dashboards, unlike the identical failure mode in either sibling cache.

Existing test confirms the current (gap) behavior is deliberate-looking but unmetriced:
test/unit/selfhost-redis-cache.test.ts:124 — "returns false without incrementing when Redis get
throws" — codifies today's no-metric-on-error behavior; this is the test to update once the metric
call is added.

Requirements

• Add an error-result metric call in both isWebhookDeliveryDuplicate's and
  rememberWebhookDelivery's catch blocks in src/selfhost/redis-cache.ts, mirroring the exact
  pattern redis-token-cache.ts/redis-response-cache.ts already use (a named metric-recording
  function called with an "error" result, not a bare incr call — check the actual current metric
  function signature in this file/its imports before writing the patch).
• Do not change the existing fail-open behavior (returning false/no-op on error) — this issue is
  scoped to adding observability, not changing the error-handling contract.
• Do not touch redis-token-cache.ts or redis-response-cache.ts — they are already correct and are
  the reference pattern to match.

Deliverables

• isWebhookDeliveryDuplicate and rememberWebhookDelivery in src/selfhost/redis-cache.ts both
  record an error metric when their Redis call throws, matching the sibling caches' pattern.
• test/unit/selfhost-redis-cache.test.ts:124's existing test ("returns false without incrementing
  when Redis get throws") is updated to assert the new error-metric call fires, alongside the
  existing fail-open-return assertion.

Test Coverage Requirements

This repo's Codecov patch gate is 99%+ of changed lines and branches (src/** is covered). The new
metric-call lines in both catch blocks must be exercised by the updated test above (and an equivalent
new/updated test for rememberWebhookDelivery's catch block if one doesn't already exist).

Expected Outcome

A sustained Redis outage affecting webhook-delivery dedup is now visible in the same metric surface
that already catches token-cache and response-cache Redis outages — no more blind spot specific to
this one cache.

Links & Resources

• src/selfhost/redis-cache.ts:18-38 — the two functions to fix
• src/selfhost/redis-token-cache.ts — reference pattern (error metric in get/set)
• src/selfhost/redis-response-cache.ts — reference pattern (error metric in get/set)
• test/unit/selfhost-redis-cache.test.ts:124 — the existing test to update

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JSO-5868