Add the HTTP hardening the policy requires before 1.0 (closes #108)

Every answer carries the six security headers REPO_POLICIES.md lists.
The Content-Security-Policy allows inline content only with a nonce made
for that answer; the pages put it on their style and script blocks and
drop their four style attributes.

The request log and /ip/ take a forwarded address only from a proxy
listed in TRUSTED_PROXIES, which stops the start when set but invalid:
the last X-Forwarded-For entry that is not a trusted proxy. X-Real-IP
and True-Client-IP are no longer read.

The JSON routes and the health check send fixed messages in place of
database, parser and resolver errors, and log the detail.

Model: opus-5-5

README.md

@@ -54,8 +54,9 @@ DEBUG=routewatch ./routewatch
   picked at random, each with its newest route (HTML)
 - `GET /prefixlength6/{length}` - The same for IPv6
 - `GET /ip/{addr}` - The same JSON answer as `/api/v1/ip/{ip}`
-- `GET /ip/` - The same, for the caller's own address, or the one a proxy names
-  in `True-Client-IP`, `X-Real-IP` or `X-Forwarded-For`
+- `GET /ip/` - The same, for the caller's own address: the address the request
+  came from or, when that is a trusted proxy (`TRUSTED_PROXIES`, see
+  Configuration), the client address in its `X-Forwarded-For`
 
 ### API v1
 
@@ -84,7 +85,17 @@ and 4 seconds for the database reads behind `/status.json` and `/api/v1/stats`.
 An AS number, prefix or mask length that does not parse answers 400, as does a
 mask length out of range for its IP version. An AS the database does not hold,
 or a prefix with no live route, answers 404. The HTML pages give these errors as
-plain text.
+plain text. A 500 answer says only `Internal server error`, and a host name that
+does not resolve only `could not resolve host name`; the detail goes to the log.
+
+Every answer carries `Strict-Transport-Security`, `Content-Security-Policy`,
+`X-Frame-Options`, `X-Content-Type-Options`, `Referrer-Policy` and
+`Permissions-Policy`. The Content-Security-Policy lets a page load content only
+from the server, and does not allow inline content with `'unsafe-inline'`: the
+pages keep their CSS and JavaScript in inline `<style>` and `<script>` blocks,
+so each answer's policy names a random nonce made for that answer, and the
+browser applies or runs only the blocks that carry it. The pages therefore use
+no `style` attributes, which a nonce cannot cover.
 
 An origin AS of 0 (`origin_asn` in a route, `asn` in an IP lookup, the key `0`
 in a prefix's `origins`) means the route has no known origin: its AS path ends
@@ -247,10 +258,20 @@ Configuration is handled via environment variables and OS-specific paths:
 | `XDG_DATA_HOME`    | `/var/lib` (in the Docker image) | Base of the state directory; must be an absolute path       |
 | `GOMEMLIMIT`       | `1536MiB` (in the Docker image)  | Go soft memory limit; see Memory                            |
 | `MALLOC_ARENA_MAX` | `2` (in the Docker image)        | glibc malloc arena cap, a positive whole number; see Memory |
+| `TRUSTED_PROXIES`  | (empty)                          | Addresses and CIDR ranges of trusted proxies; see below     |
 
 A variable that is set to an invalid value stops the start with an error and a
 non-zero exit. An empty variable counts as unset.
 
+`TRUSTED_PROXIES` lists the reverse proxies whose `X-Forwarded-For` the server
+believes, comma-separated, such as `10.0.0.0/8,192.0.2.1`. For a request from
+one of them, the request log and `/ip/` use the last address in
+`X-Forwarded-For` that is not a trusted proxy: each proxy adds the address it
+received the request from to the end, so the entries before it are whatever the
+client sent. For a request from any other address they use the address the
+request came from, whatever forwarding headers it carries. `X-Real-IP` and
+`True-Client-IP` are not read.
+
 State directory (database location):
 
 - macOS: `~/Library/Application Support/berlin.sneak.app.routewatch/`
@@ -336,9 +357,12 @@ What the [upaas](https://git.eeqj.de/sneak/upaas) app needs:
 
 - Container port: `8080`.
 - Volume: one, at container path `/var/lib/berlin.sneak.app.routewatch`.
-- Environment: nothing is required. Leave `XDG_DATA_HOME`, `GOMEMLIMIT` and
-  `MALLOC_ARENA_MAX` at the image's values. `DEBUG=routewatch` is optional and
-  adds the `System stats` memory line to the log.
+- Environment: `TRUSTED_PROXIES`, set to the address the reverse proxy in front
+  of the app connects from, as the container sees it; without it the request log
+  and `/ip/` give the proxy's address for every request. Nothing else is needed:
+  leave `XDG_DATA_HOME`, `GOMEMLIMIT` and `MALLOC_ARENA_MAX` at the image's
+  values. `DEBUG=routewatch` is optional and adds the `System stats` memory line
+  to the log.
 - Memory Limit: `5g`, the 5 GiB limit from Memory above. upaas sets no swap
   limit, so on a host with swap Docker allows the same amount of swap again.
 - Health check: the image's `HEALTHCHECK` requests