Sentry: self-host + hosted wiring + docs

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Part of #998.

Context

Make Sentry trivial for self-hosters (their own DSN) and set up the hosted side (gittensory's DSN + alerting).

Deliverables

• Self-host: SENTRY_DSN documented as an optional env var in the self-host docs + compose / .env example; off by default.
• Hosted: gittensory's DSN configured as a secret; environments (prod / preview); release health.
• Alerting rules (error spike, DLQ growth, AI-quota exhaustion) + an owner notification path.
• (Optional) link Sentry issue/health into the dashboard view (ties to Dashboard + observability for self-hosted instances #982).

Requirements / acceptance

• A self-hoster enables Sentry with one env var and sees their own errors/traces.
• Hosted alerts fire on a representative failure.

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Closing as completed (2026-07-01 issue audit): Self-host Sentry wiring/docs/alerting shipped (#1547/#1743/#1860/#1721); hosted-DSN half stale per pivot.