feat(sentry): remove the startup probe

[SyniRon]

Closes #259

What changed

Boot no longer sends a canary event to Sentry or waits on a flush window. SetupSentry keeps its signature and return values. It still runs sentryDialCheck, and it now prints the "Sentry error capture enabled" line whenever init succeeded.

• rest/sentry.go: the probe call and its gate on the enabled line are gone. The file header now describes the two boot pieces that remain (dial check, shutdown flush).
• rest/sentry_boot.go: sentryStartupProbe and sentryStartupProbeTimeout are deleted. The sentryDialCheck doc comment no longer justifies itself by what the probe could not see.
• rest/sentry_boot_test.go: one new test pins the boot contract. With a DSN whose host refuses connections and the transport seam swapped for a capture mock, SetupSentry returns true, the mock holds zero events, the Info log has the enabled line, and the Warn log names the host. The four probe tests and the probe-only helpers drainTransport and bindDrainClient are deleted.
• rest/sentry_test.go: transportMock.reset() and its call in enableSentry are deleted. Every existing test that counts events now also asserts, by construction, that boot sent nothing.
• servers/server.go: the boot-delay comment drops from about 8s to 3s.

Why

The probe's flush proved queue drain, not delivery. With a refused ingest host it reported success in milliseconds while sentryDialCheck already named the host as unreachable. Every container start also cost Sentry one event, and the probe's stall test was the flake in #179.

For the reviewer

• The zero-events assertion is deterministic. With a custom Transport, sentry-go v0.49.0 skips its async telemetry processor and calls Transport.SendEvent on the capturing goroutine, so a re-added boot event lands in the mock before SetupSentry returns.
• The test pins the substring "Sentry error capture enabled" on purpose. The issue names it as the contract, and this package already treats operator-facing boot log lines as contract.
• Out of scope and untouched: sentryDialCheck itself, FlushSentryOnShutdown, watchShutdown, the middleware, SENTRY_DEBUG, goldens and the spec.

Verification

• grep -ri "startup probe\|startup-probe\|StartupProbe" --include='*.go' . returns nothing.
• go build ./..., go vet ./..., go test ./... pass.
• go test -race ./rest ./servers passes.

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