feat(metrics): emit a webhook-dedup hit counter from the Redis delivery cache

[JSONbored]

checkAndMarkDelivery (src/selfhost/redis-cache.ts:30) short-circuits duplicate GitHub webhook redeliveries but records no metric on the dedup path, so operators can't quantify how many redeliveries Redis is absorbing (server.ts:722 has gittensory_webhook_dedup_total only for the in-process transient cache, not this Redis path). Add a gittensory_webhook_dedup_total{backend="redis"} increment when a delivery is already-seen.

Deliverables

• Import incr from ./metrics into src/selfhost/redis-cache.ts
• Increment gittensory_webhook_dedup_total{backend="redis"} inside checkAndMarkDelivery when seen is truthy (before returning true)
• Do not increment on the swallowed-Redis-error path (returns false)
• Unit tests: already-seen increments once, first-time does not increment, Redis-error path does not increment

References

• src/selfhost/redis-cache.ts:30
• src/server.ts:722
• src/selfhost/metrics.ts:48

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size: XS · gittensor:bug

[linear-code]

JSO-2192