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

internal/templates/prefix_length.html

@@ -4,7 +4,7 @@
     <meta charset="UTF-8">
     <meta name="viewport" content="width=device-width, initial-scale=1.0">
     <title>Prefixes with /{{ .MaskLength }} - RouteWatch</title>
-    <style>
+    <style nonce="{{.CSPNonce}}">
         * {
             box-sizing: border-box;
         }