Deepen run transition metrics

[darthfork]

Summary

• add a run-transition Module that owns stored run lifecycle transitions, stale/duplicate rejection, gauge deltas, and duration observation
• keep workflow/job payload handlers as event-specific adapters that pass normalized facts into the transition Module
• deepen integration coverage for Candidate 1 by asserting delivery-store failures return 500 and do not enqueue/process events
• deepen integration coverage for Candidate 2 by driving queued -> in_progress -> completed workflow_run events through /webhook and verifying balanced gauges plus duration metrics

TDD notes

• RED: added interface-level runTransitionProcessor tests first; they failed on the missing Module Interface
• GREEN: introduced the Module and wired workflow/job metric updates through it
• REFACTOR: moved run transition types/recording out of github.go and removed shallow helper orchestration