server: report HTTP handler panics to Sentry (closes #107)

DNSWATCHER_SENTRY_DSN was read but never used. This ports the Sentry
integration from gohttpserver with sentry-go v0.49.0. The server's start
hook calls sentry.Init when the DSN is set; a DSN Sentry cannot parse
fails the hook, so startup stops with the parse error. sentryhttp, with
Repanic, reports handler panics and passes them on to chi's Recoverer.
Shutdown sends queued reports once the HTTP server has stopped. The
client uses the older transport (DisableTelemetryBuffer): with the
default one, Flush can return before sending a report made just before
it. The DSN is checked at server start, not in config, so the config
test is unchanged. sentry-go raises several golang.org/x modules.

Model: opus-5-5

README.md

@@ -352,6 +352,13 @@ either is set to anything else, including a bare number or a zero or negative
 duration, dnswatcher refuses to start with an error naming the variable and
 the value.
 
+**`DNSWATCHER_SENTRY_DSN` reports crashes in HTTP requests to Sentry.** When it
+is set, a panic in an HTTP request handler is sent to Sentry, and the request
+still gets a `500 Internal Server Error` answer. Nothing else is sent to Sentry:
+DNS, port and TLS problems are reported as notifications. A value Sentry cannot
+parse stops dnswatcher at startup. At shutdown, reports not yet sent are sent,
+waiting at most 2 seconds.
+
 ### Example `.env`
 
 ```sh