Warn when production shares one rate-limit bucket (closes #149)

With TRUSTED_PROXIES empty, every rate limiter keys on the connecting
peer. Production runs behind a TLS-terminating reverse proxy, so the
peer is that proxy for every request and all clients share one bucket
per limit. For the login limiter that means any remote client sending
five POSTs a minute holds the only administrative login at HTTP 429.

The empty default is correct — trusting forwarded headers from
arbitrary peers lets any client choose its own bucket — so this makes
the consequence visible rather than changing the keying, the limits or
the default:

- config logs a WARN at startup when the environment is prod and
  TRUSTED_PROXIES is empty, naming the variable, the shared bucket and
  the deniable admin login.
- The security-feature bullet's "per IP" login claim is now conditional
  on TRUSTED_PROXIES, which is the only case where it holds.
- The rate-limiting section separates the receiver case (sharing costs
  throughput, the safe direction) from the login case (sharing costs
  availability of the only admin path, not safe).
- The trusted-proxies configuration section states the consequence and
  names TRUSTED_PROXIES as the remedy.

internal/config/export_test.go

@@ -1,9 +1,26 @@
 package config
 
+import "log/slog"
+
 // This file exposes the unexported environment parsing helpers to
 // the external config_test package so each helper can be covered by
 // its own table-driven test without weakening the package API.
 
+// WarnSharedRateLimitBucketForTest loads a Config from the current
+// environment and emits its startup warnings to log. The real logger
+// writes to stdout, so this lets the warning's firing condition be
+// asserted against a handler the test controls.
+func WarnSharedRateLimitBucketForTest(log *slog.Logger) error {
+	c, err := loadFromEnv()
+	if err != nil {
+		return err
+	}
+
+	c.warnSharedRateLimitBucket(log)
+
+	return nil
+}
+
 // EnvBoolForTest exposes envBool.
 func EnvBoolForTest(key string, defaultValue bool) (bool, error) {
 	return envBool(key, defaultValue)