Unset TRUSTED_PROXIES trusts RFC 1918, empty trusts none (closes #129)

The backend's own TRUSTED_PROXIES now follows the ruling for every app:
unset, it trusts 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16, and no
longer loopback; a list replaces that; set and empty, it trusts no
proxy.

viper reads an empty environment variable as unset, so an empty value
got the default. config.New now reads this one setting with
os.LookupEnv first. viper's AllowEmptyEnv would apply to every setting,
so an empty PORT or DEBUG would stop the start instead of counting as
unset, as README.md says it does.

bin/entrypoint.sh still starts the backend with
TRUSTED_PROXIES=127.0.0.1/32.

Model: opus-5-5

TODO.md

@@ -21,6 +21,14 @@ Run `make frontend-viewport-test` in CI as its own step; it is not part of
 
 # Completed Steps
 
+- 2026-10-14: unset, the backend's own `TRUSTED_PROXIES` trusts `10.0.0.0/8`,
+  `172.16.0.0/12` and `192.168.0.0/16`, and no longer loopback; a list replaces
+  that; set and empty, it trusts no proxy, where it used to get the default, as
+  viper reads an empty environment variable as unset
+  ([#129](https://git.eeqj.de/sneak/netwatch/issues/129)). A value it cannot
+  parse still stops the start, and the image still starts the backend with
+  `TRUSTED_PROXIES=127.0.0.1/32`. Tests load the config with it unset, empty and
+  set to a list
 - 2026-10-14: while report files cannot be written, `POST /api/v1/reports`
   answers 500, and storing recovers once writing works again, with no restart
   ([#123](https://git.eeqj.de/sneak/netwatch/issues/123)). A failed write puts