feat: 建立持久任务、租约与运行可观测性

[mahoshojoHCG]

Summary

• persist download completion as PostgreSQL durable jobs with renewable leases, bounded retries, dead-letter administration, and task-dashboard controls
• bound production in-process queues and use database-backed scheduling leases so multi-instance periodic work has one active executor with expiry takeover
• add dependency-aware readiness, dependency-free liveness, Prometheus/OpenTelemetry instrumentation, sensitive-tag redaction, migrations, and operations documentation
• preserve the download submission/cancellation lease saga and release-upgrade activation ordering while integrating the latest main branch

Boundaries

• This PR does not add a concrete Webhook or Web Push transport. The default notifier is an extension point; adapters must persist the stable event ID before external effects.
• Provider-neutral AI updates currently expose no token-usage data, so AI calls have latency/tracing coverage but no token counter.
• No tests, test-only scripts, workflow assertions, or other regression checks are added by this PR.

Closes #14

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-08-29T15:40:19.215243Z	0550893	PR opened

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review" or "@codex security review".

Codex reacts with 👀 while any review is running, comments if it has suggestions, and reacts with 👍 once all reviews finish with no findings.

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 055089304b

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".