Unset TRUSTED_PROXIES trusts the RFC 1918 ranges (closes #119)

With TRUSTED_PROXIES unset, bin/entrypoint.sh gives nginx one
set_real_ip_from line for each of 10.0.0.0/8, 172.16.0.0/12 and
192.168.0.0/16, as sneak ruled on 2026-09-22 for every app. A list
replaces them, and set and empty trusts no proxy; each entry is checked
as before. The backend inside the container still trusts only
127.0.0.1/32. README.md, configs/nginx.conf and the comment in
script/frontend-viewport-test say the new default.

Model: opus-5-5

README.md

@@ -412,9 +412,10 @@ address nginx passes on and no other. The image:
 
 - Listens on port 8080 by default (override with `PORT` env var)
 - Takes the client address from `X-Forwarded-For` only on requests from the
-  reverse proxies named in `TRUSTED_PROXIES`, and by default from none.
-  `bin/entrypoint.sh` checks each entry with `netwatch-server check-cidr`, which
-  parses it as the backend parses its own `TRUSTED_PROXIES`
+  reverse proxies named in `TRUSTED_PROXIES`, by default from any address in
+  `10.0.0.0/8`, `172.16.0.0/12` or `192.168.0.0/16`. `bin/entrypoint.sh` checks
+  each entry with `netwatch-server check-cidr`, which parses it as the backend
+  parses its own `TRUSTED_PROXIES`
 - Sends access logs to stdout
 - Caches static assets with immutable headers
 - Sends the security headers `REPO_POLICIES.md` requires on every response, as
@@ -433,9 +434,9 @@ What the [upaas](https://git.eeqj.de/sneak/upaas) app for netwatch needs:
 
 - **Port:** container port `8080`.
 - **Volume:** container path `/data`; the reports are kept in `/data/reports`.
-- **Environment variables:** none is required. An empty one counts as unset, and
-  one set to a value netwatch cannot use stops the container at start, with the
-  reason in its log.
+- **Environment variables:** none is required. An empty one counts as unset,
+  except `TRUSTED_PROXIES`, and one set to a value netwatch cannot use stops the
+  container at start, with the reason in its log.
     - `PORT`, default `8080`: the container port, from 1 to 65535. `8081` cannot
       be used: the backend listens on it inside the container
     - `REPORTS_PER_MINUTE`, default `60`: reports each client address may send a
@@ -449,16 +450,17 @@ What the [upaas](https://git.eeqj.de/sneak/upaas) app for netwatch needs:
       in: `/data` or a path below it, with no `.` or `..` part and no extra `/`.
       The container also stops if the path goes through a symbolic link that
       leads out of `/data` or is written as a full path
-    - `TRUSTED_PROXIES`, default empty: set it to the address the reverse proxy
-      in front of the container connects from, as an IP address or CIDR; several
-      are separated by commas. nginx takes the client address from
-      `X-Forwarded-For` only on a request from one of them, and the rate limit
-      counts that address. Unset, `X-Forwarded-For` is ignored and every client
-      behind the proxy shares the proxy's one allowance of `REPORTS_PER_MINUTE`.
-      Name only addresses nothing but the proxy connects from: any client that
-      connects from one can write its own `X-Forwarded-For`, and through a port
-      Docker publishes, every client may connect from the Docker network's
-      gateway, such as `172.17.0.1`.
+    - `TRUSTED_PROXIES`, default `10.0.0.0/8,172.16.0.0/12,192.168.0.0/16`: the
+      addresses the reverse proxy in front of the container connects from, as IP
+      addresses or CIDRs separated by commas. nginx takes the client address
+      from `X-Forwarded-For` only on a request from one of them, and the rate
+      limit counts that address. A list replaces the default. Set and empty,
+      `X-Forwarded-For` is ignored and every client behind the proxy shares the
+      proxy's one allowance of `REPORTS_PER_MINUTE`. Any client that connects
+      from one of these addresses can write its own `X-Forwarded-For`, and
+      through a port Docker publishes, every client may connect from the Docker
+      network's gateway, such as `172.17.0.1`, which the default includes; where
+      that matters, name only the addresses nothing but the proxy connects from.
     - `METRICS_USERNAME` and `METRICS_PASSWORD`, default empty: with both set,
       the backend records Prometheus metrics of its requests and serves them at
       `/metrics` on the container port, to requests with this user name and