metrics: rate limit /metrics per client address before Basic Auth (closes #101)

/metrics is behind a password, and REPO_POLICIES.md requires rate
limiting on password logins. Each client address may now send it 30
requests a minute, counted by httprate before Basic Auth, so failed
logins use up the allowance and a request over it gets 429 without
the password being checked. The address is the one the existing
trusted-proxy logic in internal/middleware works out, with IPv6
addresses grouped by /64; an IPv4 address a proxy reports in
IPv6-mapped form counts as the plain IPv4 address. A Prometheus
server scraping every 15 seconds sends 4 requests a minute.

Model: opus-5-5

README.md

@@ -267,7 +267,7 @@ internal/
   logger/logger.go             slog structured logging (TTY detection)
   healthcheck/healthcheck.go   Health check service
   middleware/middleware.go      HTTP middleware (logging, CORS, security
-                               headers, metrics auth)
+                               headers, metrics auth and rate limit)
   handlers/handlers.go         HTTP request handlers
   server/
     server.go                  HTTP server lifecycle
@@ -335,6 +335,15 @@ is a misconfiguration, so dnswatcher fails fast with a clear error message
 rather than running silently. Set `DNSWATCHER_TARGETS` to a comma-separated
 list of DNS names before starting.
 
+**`/metrics` is rate limited.** Each client address may send it 30 requests a
+minute, failed logins included; beyond that it answers `429 Too Many Requests`
+without checking the password. A Prometheus server scraping every 15 seconds
+sends 4 a minute. IPv6 addresses in one /64 count as one client. When the
+request comes from a private or loopback address, such as a reverse proxy's,
+the client address is taken from the `X-Real-IP` or `X-Forwarded-For` header
+the proxy sets; a proxy that sets neither makes all its clients share one
+allowance.
+
 ### Example `.env`
 
 ```sh