fix: add jitter to API retry backoff

[poaspergillus]

Summary

• Add full jitter to API retry exponential backoff.
• Add jittered recovery deadlines to the circuit breaker.
• Add/update focused unit tests for retry and recovery jitter.

Testing

• Focused Vitest tests: 28/28 passed.
• Type-check: passed.
• Prettier: passed.
• ESLint on changed files: 0 errors.
• git diff --check: passed.
• Full test suite was attempted.

Closes #1164

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