trusted proxy, TOTP replay protection, new tests

- Trusted proxy config option for proxy-aware IP extraction
  used by rate limiting and audit logs; validates proxy IP
  before trusting X-Forwarded-For / X-Real-IP headers
- TOTP replay protection via counter-based validation to
  reject reused codes within the same time step (±30s)
- RateLimit middleware updated to extract client IP from
  proxy headers without IP spoofing risk
- New tests for ClientIP proxy logic (spoofed headers,
  fallback) and extended rate-limit proxy coverage
- HTMX error banner script integrated into web UI base
- .gitignore updated for mciasdb build artifact

Security: resolves CRIT-01 (TOTP replay attack) and
DEF-03 (proxy-unaware rate limiting); gRPC TOTP
enrollment aligned with REST via StorePendingTOTP

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/config/config_test.go

@@ -210,6 +210,40 @@ threads = 4
 	}
 }
 
+// TestTrustedProxyValidation verifies that trusted_proxy must be a valid IP.
+func TestTrustedProxyValidation(t *testing.T) {
+	tests := []struct {
+		name        string
+		proxy       string
+		wantErr     bool
+	}{
+		{"empty is valid (disabled)", "", false},
+		{"valid IPv4", "127.0.0.1", false},
+		{"valid IPv6 loopback", "::1", false},
+		{"valid private IPv4", "10.0.0.1", false},
+		{"hostname rejected", "proxy.example.com", true},
+		{"CIDR rejected", "10.0.0.0/8", true},
+		{"garbage rejected", "not-an-ip", true},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			cfg, _ := Load(writeTempConfig(t, validConfig()))
+			if cfg == nil {
+				t.Fatal("baseline config load failed")
+			}
+			cfg.Server.TrustedProxy = tc.proxy
+			err := cfg.validate()
+			if tc.wantErr && err == nil {
+				t.Errorf("expected validation error for proxy=%q, got nil", tc.proxy)
+			}
+			if !tc.wantErr && err != nil {
+				t.Errorf("unexpected error for proxy=%q: %v", tc.proxy, err)
+			}
+		})
+	}
+}
+
 func TestDurationParsing(t *testing.T) {
 	var d duration
 	if err := d.UnmarshalText([]byte("1h30m")); err != nil {