feat(dead-letter): add transient auto-requeue

[pureliture]

What

• Add bounded dead-letter auto-requeue recovery for transient verifier dead-letter jobs.
• Split dead-letter classification into terminal reason and root error class.
• Persist autoRequeueCount and enforce one automatic requeue per job.
• Add guarded NoSQL conditional recovery behavior.
• Add conservative personal systemd service/timer units without enabling them by default.

Why

Operators need a safe recovery path for verifier timeout, transport, and lease-expiry backlog after overload without introducing SQS, LocalStack, Kafka-style offsets, outbox storage, new tables, or new GSIs.

Safety

• Dry-run by default; mutation requires explicit apply mode.
• Defaults to verify jobs.
• Only transient root error classes are eligible.
• Cooldown-gated, limit-gated, page-capped, compare-and-set guarded, and one-shot.
• Non-transient, malformed, unknown, scanner-runtime, and retry-budget-only failures remain excluded from automatic requeue.

Verification

• ruff check on changed files passed.
• Whitespace diff check passed.
• Related regression tests passed.
• Operational proof covered a synthetic dead-letter verify job, dry-run candidate detection, apply recovery to pending, one-shot skip on a repeated dead-letter, and a one-shot user systemd service run without enabling the timer.

Closes #103

[gemini-code-assist]

Code Review

This pull request introduces a conservative, timer-ready dead-letter auto-requeue feature to automatically recover transient verifier failures. It includes new systemd user service and timer units, design and requirements documentation, a new CLI command, and NoSQL database updates to track auto-requeue counts and perform conditional updates. Feedback highlights a potential issue in the DynamoDB conditional write expression when last_error is None (which can cause valid jobs to be skipped), as well as a missing keyword-only argument separator in the storage protocol signature that causes type-checking mismatches.

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