Report handler panics to Sentry when SENTRY_DSN is set (closes #95)

With SENTRY_DSN set, the server initialises sentry-go with the release
netwatch-server-<version>, adds the sentryhttp middleware with Repanic
as the last router-wide middleware, after the timeout, and flushes
Sentry for 2 seconds on shutdown. A DSN Sentry refuses stops the start
with an error naming SENTRY_DSN. With it empty, nothing is set up.

The metrics middleware stays on the matched routes only, so it runs
inside the Sentry middleware rather than before it.

Model: opus-5-5

backend/go.sum

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