feat(#123): add IP-level SSRF defense to Gitea client and action

## Changes

### Go: IP-level SSRF protection in gitea.Client (primary defense)
- Add gitea/ipcheck.go with IsBlockedIP() covering all blocked CIDR ranges:
  loopback (127.0.0.0/8, ::1), RFC1918 (10/8, 172.16/12, 192.168/16),
  link-local (169.254/16, fe80::/10), ULA (fc00::/7), CGN (100.64/10),
  multicast, reserved, and unspecified ranges.
- IPv6-mapped IPv4 addresses (::ffff:x.x.x.x) are normalized before checking.
- Add safeDialContext to gitea.Client: resolves DNS, rejects any IP in a
  blocked CIDR, then dials the resolved IP directly to narrow the DNS rebinding
  window. NewClient now uses this safe transport by default.
- Add WithUnsafeDialer() for test code using httptest.Server (127.0.0.1).
- Update NewTestClient helper in export_test.go for all gitea unit tests.
- Update SetHTTPClient(nil) to restore the safe transport (not the plain one).

### Go: validate-url subcommand (defense-in-depth for future bash callers)
- Add 'review-bot validate-url <url>' subcommand: validates https scheme,
  no user-info, resolves hostname, rejects any blocked IP.
- Exit 0=safe, 1=blocked, 2=validation error/dns failure.
- Add outWriter/errWriter vars to main.go for testable output capture.

### action.yml: Python3 IP check in 'Determine version' step
- After the https scheme validation, resolve SERVER_URL hostname with
  socket.getaddrinfo and reject any result where
  ipaddress.ip_address(ip).is_private/is_loopback/is_link_local/etc. is true.
- python3 is required on ubuntu-* runners (noted in existing comments).
- Covers the version-check curl that sends ACTION_TOKEN to SERVER_URL.
- SERVER_URL for install-step curls is covered by the same pre-check.

### Tests
- gitea/ipcheck_test.go: 30+ cases covering all blocked families + public IPs
- gitea/client_test.go: safe transport presence, WithUnsafeDialer, SSRF blocking
- cmd/review-bot/validateurl_test.go: scheme validation, user-info, exit codes

Closes #123

gitea/ipcheck_test.go

@@ -0,0 +1,127 @@
+package gitea
+
+import (
+	"net"
+	"testing"
+)
+
+func TestIsBlockedIP(t *testing.T) {
+	blocked := []struct {
+		name string
+		ip   string
+	}{
+		// IPv4 loopback
+		{"loopback 127.0.0.1", "127.0.0.1"},
+		{"loopback 127.0.0.2", "127.0.0.2"},
+		{"loopback 127.255.255.255", "127.255.255.255"},
+		// IPv4 unspecified
+		{"unspecified 0.0.0.0", "0.0.0.0"},
+		{"unspecified 0.1.2.3", "0.1.2.3"},
+		// RFC1918
+		{"RFC1918 10.0.0.1", "10.0.0.1"},
+		{"RFC1918 10.255.255.255", "10.255.255.255"},
+		{"RFC1918 172.16.0.1", "172.16.0.1"},
+		{"RFC1918 172.31.255.255", "172.31.255.255"},
+		{"RFC1918 192.168.0.1", "192.168.0.1"},
+		{"RFC1918 192.168.255.255", "192.168.255.255"},
+		// Link-local (APIPA / AWS metadata)
+		{"link-local 169.254.0.1", "169.254.0.1"},
+		{"link-local 169.254.169.254", "169.254.169.254"},
+		// Shared address space (carrier-grade NAT)
+		{"CGN 100.64.0.1", "100.64.0.1"},
+		{"CGN 100.127.255.255", "100.127.255.255"},
+		// Multicast
+		{"multicast 224.0.0.1", "224.0.0.1"},
+		{"multicast 239.255.255.255", "239.255.255.255"},
+		// Reserved
+		{"reserved 240.0.0.1", "240.0.0.1"},
+		{"broadcast 255.255.255.255", "255.255.255.255"},
+		// IPv6 loopback
+		{"IPv6 loopback ::1", "::1"},
+		// IPv6 unspecified
+		{"IPv6 unspecified ::", "::"},
+		// IPv6 link-local
+		{"IPv6 link-local fe80::1", "fe80::1"},
+		{"IPv6 link-local fe80::dead:beef", "fe80::dead:beef"},
+		// IPv6 ULA
+		{"IPv6 ULA fc00::1", "fc00::1"},
+		{"IPv6 ULA fd00::1", "fd00::1"},
+		// IPv6 multicast
+		{"IPv6 multicast ff02::1", "ff02::1"},
+	}
+
+	for _, tc := range blocked {
+		t.Run(tc.name, func(t *testing.T) {
+			ip := net.ParseIP(tc.ip)
+			if ip == nil {
+				t.Fatalf("failed to parse IP %q", tc.ip)
+			}
+			if !IsBlockedIP(ip) {
+				t.Errorf("IsBlockedIP(%q) = false, want true", tc.ip)
+			}
+		})
+	}
+
+	allowed := []struct {
+		name string
+		ip   string
+	}{
+		{"public 8.8.8.8", "8.8.8.8"},
+		{"public 1.1.1.1", "1.1.1.1"},
+		{"public 198.51.100.1", "198.51.100.1"},     // TEST-NET-2, but not blocked — public-looking
+		{"public 151.101.1.1", "151.101.1.1"},        // Fastly
+		{"public IPv6 2001:4860:4860::8888", "2001:4860:4860::8888"}, // Google DNS
+		{"public IPv6 2606:4700:4700::1111", "2606:4700:4700::1111"}, // Cloudflare DNS
+	}
+
+	for _, tc := range allowed {
+		t.Run(tc.name, func(t *testing.T) {
+			ip := net.ParseIP(tc.ip)
+			if ip == nil {
+				t.Fatalf("failed to parse IP %q", tc.ip)
+			}
+			if IsBlockedIP(ip) {
+				t.Errorf("IsBlockedIP(%q) = true, want false", tc.ip)
+			}
+		})
+	}
+}
+
+func TestIsBlockedIPv6MappedIPv4(t *testing.T) {
+	// ::ffff:192.168.1.1 is an IPv6-mapped IPv4 address — should be blocked as RFC1918.
+	// Construct it manually as a 16-byte IP.
+	mapped := net.IP{0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0xff, 0xff, 192, 168, 1, 1}
+	if !IsBlockedIP(mapped) {
+		t.Errorf("IsBlockedIP(::ffff:192.168.1.1) = false, want true (IPv6-mapped IPv4 must be normalized)")
+	}
+
+	// ::ffff:8.8.8.8 — IPv6-mapped public IP — should be allowed.
+	mappedPublic := net.IP{0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0xff, 0xff, 8, 8, 8, 8}
+	if IsBlockedIP(mappedPublic) {
+		t.Errorf("IsBlockedIP(::ffff:8.8.8.8) = true, want false")
+	}
+}
+
+func TestIsBlockedIPEdgeCases(t *testing.T) {
+	// The boundary between RFC1918 and public ranges.
+	// 172.15.255.255 is NOT private (just below 172.16.0.0/12).
+	notPrivate := net.ParseIP("172.15.255.255")
+	if IsBlockedIP(notPrivate) {
+		t.Errorf("IsBlockedIP(172.15.255.255) = true, want false (outside 172.16.0.0/12)")
+	}
+	// 172.32.0.0 is NOT private (just above 172.31.255.255).
+	notPrivate2 := net.ParseIP("172.32.0.0")
+	if IsBlockedIP(notPrivate2) {
+		t.Errorf("IsBlockedIP(172.32.0.0) = true, want false (outside 172.16.0.0/12)")
+	}
+	// CGN: 100.63.255.255 is NOT in 100.64.0.0/10.
+	notCGN := net.ParseIP("100.63.255.255")
+	if IsBlockedIP(notCGN) {
+		t.Errorf("IsBlockedIP(100.63.255.255) = true, want false (outside 100.64.0.0/10)")
+	}
+	// CGN: 100.128.0.0 is NOT in 100.64.0.0/10.
+	notCGN2 := net.ParseIP("100.128.0.0")
+	if IsBlockedIP(notCGN2) {
+		t.Errorf("IsBlockedIP(100.128.0.0) = true, want false (outside 100.64.0.0/10)")
+	}
+}