Move GitHub-webhook rate-limit buckets from IP-keyed to installation-keyed

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Problem: Rate-limit buckets for GitHub-originated traffic are keyed by connecting IP address today — correct when every self-hoster has their own IP, but a shared multi-customer egress path would collide multiple customers' traffic into the same bucket.

Area: ORB / Cloud

Proposal: Re-key these buckets by installation/customer identity rather than IP address for any centrally-hosted path.

Deliverables:

• Installation-keyed rate limiting for the relevant webhook/token/relay endpoints.

Acceptance criteria:

• Two simulated customers behind the same egress IP don't contend for the same rate-limit bucket.

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JSO-3551