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/README.md

@@ -90,6 +90,7 @@ project layout:
 | `CORS_ALLOWED_ORIGINS` | empty                | Comma-separated origins whose pages may call the API; see [CORS](#cors)                                  |
 | `METRICS_USERNAME`     | empty                | Basic auth user name for `/metrics`; see [Metrics](#metrics)                                             |
 | `METRICS_PASSWORD`     | empty                | Basic auth password for `/metrics`; see [Metrics](#metrics)                                              |
+| `SENTRY_DSN`           | empty                | DSN of the Sentry project to send errors to; see [Sentry](#sentry)                                       |
 
 `TRUSTED_PROXIES` defaults to
 `127.0.0.1/32,::1/128,10.0.0.0/8,172.16.0.0/12,192.168.0.0/16`. The loopback
@@ -198,6 +199,16 @@ is recorded and `/metrics` answers 404. One without the other stops the server
 from starting, with an error naming both; so does a `METRICS_USERNAME`
 containing `:`, which basic auth cannot carry, with an error naming it.
 
+### Sentry
+
+With `SENTRY_DSN` set, the server sends its errors to that Sentry project: each
+panic in a handler is reported there, under the release `netwatch-server-`
+followed by the server's version, and the request still gets 500 from the
+server's panic recovery. On shutdown the server waits up to 2 seconds for Sentry
+to finish sending. A DSN Sentry refuses stops the server from starting, with an
+error naming `SENTRY_DSN`. With it empty, Sentry is not set up, and nothing is
+sent to it.
+
 ## TODO
 
 - Add integration test that POSTs a report and verifies the compressed output