feat(observability): expand self-host Sentry context and monitors

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Parent: #1819

Context

The self-host runtime has optional Sentry wiring, release resolution, source-map support for official/self-built releases, structured-log forwarding, and selected review failure capture. The next step is broader, consistent operational context and monitor coverage for critical review-system paths.

Sentry must remain opt-in. Community self-hosters should send events only to their own DSN if they choose to enable it.

Requirements

• Keep SENTRY_DSN unset as a complete no-op with negligible overhead.
• Add safe Sentry context/tags for critical self-host engine paths: webhook ingest, queue claim/process, GitHub token/broker calls, AI provider attempts, gate/check-run publish, comment publish, scheduled sweep, and backup/relay failures.
• Add Sentry cron monitors or equivalent check-ins for recurring work where silent stoppage is worse than an exception.
• Define alert classes and thresholds that are actionable without paging on one-off fail-open noise.
• Scrub all secrets, tokens, auth headers, request bodies, raw diffs, prompts, review text, private repo config, and local auth paths.
• Keep high-cardinality identifiers out of tags unless safely truncated or intentionally fingerprinted.

Deliverables

• Sentry context/tag helper or documented taxonomy for self-host operational events.
• Monitor coverage for the scheduled loop and the highest-value recurring jobs.
• Alert/runbook documentation for dead-letter growth, check-run permission gaps, AI provider exhaustion, relay failures, backup failures, and scheduled monitor misses.
• Tests for no-op behavior, scrubbing, and representative capture sites.
• Website docs describing how self-hosters enable Sentry with their own DSN.

Acceptance criteria

• A production Sentry issue points to the affected subsystem, repo/PR when applicable, release, environment, job type, and safe reason code.
• No sensitive payloads or private config values are present in captured events.
• Operators can distinguish transient degradation from persistent failure classes.
• Sentry remains disabled by default for community self-hosters.

Validation

• Unit tests for scrubbing/tag allowlists and no-DSN behavior.
• Focused smoke test or controlled failure confirming context appears in Sentry when enabled.
• Docs build and relevant self-host observability tests pass.

Ownership

Maintainer-only. No gittensor:* labels.

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2026-07-01 audit: absorbing #1739 (browser/worker Sentry alert rules + ownership + runbooks).