Make backup terminal reports idempotent

[arjunkomath]

Summary

• atomically transition backup completion, failure, and timeout only from nonterminal states
• make the backup state machine consistently follow a first-terminal-write-wins rule
• treat repeated or late terminal reports as successful no-ops
• add deterministic Inngest event IDs to prevent duplicate delivery
• cover successful agent transitions and replay behavior with focused route tests

A timeout is terminal: if it wins first, a later completion does not replace the failure. If completion or agent-reported failure wins first, the timeout handler cannot overwrite it.

Verification

• pnpm test — 59 files, 386 tests passed
• pnpm exec vitest run tests/agent-backup-complete-route.test.ts tests/agent-backup-failed-route.test.ts — 4 tests passed
• ./node_modules/.bin/tsc --noEmit
• npx biome check --write ...

Basecamp: https://app.basecamp.com/5890597/buckets/47439993/todos/10146625754

[github-actions]

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_{Reviewed by React Doctor for commit fe820a8.}