server: a port that cannot be bound stops startup (closes #266)

The start hook now binds the listen port itself and serves on it in a
goroutine. A bind failure, such as a port already in use or a
privileged port without root, is returned to fx, which stops startup
and exits 1 with an error like "starting http server: listen tcp
:8080: bind: address already in use". Before, the bind happened inside
the goroutine and its error was only logged while the watcher kept
running without the dashboard, API or health check.

The http.Server is now built before that goroutine starts, so Shutdown
no longer reads a field another goroutine may still be writing.

TestPortInUseStopsStartup holds a port and checks that starting the
server on it fails with an error naming it.

Model: opus-5-5

TODO.md

@@ -19,6 +19,8 @@ trial run of the finished image: https://git.eeqj.de/sneak/dnswatcher/issues/149
 
 # Completed Steps
 
+- 2026-10-13: a `PORT` that cannot be bound stops startup with an error naming
+  the address, instead of running without the HTTP server (closes #266).
 - 2026-10-13: a check saving the state while the dashboard or `/api/v1/status`
   is read cannot crash the process (closes #261).
 - 2026-10-06: canonical files re-fetched from `sneak/prompts` at `dd4027b`; lint