Sentry: SDK integration + optional DSN seam (off by default)

[JSONbored]

Part of #998.

Context

Wire @sentry/cloudflare into the Worker, initialised from an optional SENTRY_DSN. No DSN ⇒ no init ⇒ zero overhead. Hosted uses gittensory's DSN; self-host uses the operator's.

Deliverables

• Add @sentry/cloudflare; init Sentry in the Worker entry (fetch / queue / scheduled), guarded by SENTRY_DSN.
• Optional bindings: SENTRY_DSN (+ SENTRY_ENVIRONMENT, SENTRY_TRACES_SAMPLE_RATE, SENTRY_RELEASE), documented as optional.
• Wrap the engine handlers so uncaught errors are captured (with minimal context — no request bodies).
• Release tagging from the deploy SHA.

Requirements / acceptance

• Unset DSN = Sentry fully inert (no network, no perf cost) — covered by a test.
• tsc --noEmit + vitest run green.