Add an egress CIDR allowlist to the SSRF guard (closes #204)

The SSRF blocklist had no escape hatch, so the thing webhooker is
mostly for — taking a public webhook and forwarding it to something
on your own network — could not be configured at all. Every private
address, Docker sibling and loopback service was permanently
unreachable as a delivery destination.

ALLOWED_EGRESS_CIDRS (default empty) names blocks that delivery
targets may reach despite the default blocklist. It is an allowlist
and only ever adds destinations: there is no boolean, and no value
disables SSRF protection wholesale. Empty, the guard behaves exactly
as before.

Link-local (169.254.0.0/16, fe80::/10) is refused before the
allowlist is consulted, so no supplied CIDR can open it — not the
exact address, not a supernet, not 0.0.0.0/0. Reaching cloud
instance metadata is credential theft rather than delivery to an
internal service.

The policy now lives in one function, Guard.checkIP, which both
target-creation validation and the delivery dialer call. The two
paths previously decided separately, which is how they came to
disagree about a destination. The guard is built once from config
and injected via fx into both the handlers and the delivery engine,
so there is a single instance and a single answer.

A set-but-unparseable value aborts startup naming the variable,
reusing the existing envPrefixList parser. A non-empty list is
logged at startup with the blocks spelled out, not counted, so the
hole is visible in the log of any deployment that has one.

Tests: an allowlisted loopback CIDR both validates and delivers to a
live server (and the same URL still fails without the allowlist); a
private address outside the listed block stays refused on both
paths; metadata stays refused under six different covering CIDRs;
public addresses are unaffected either way; and config coverage for
parsing, startup abort, and the warning's contents.

internal/delivery/ssrf_test.go

@@ -31,10 +31,10 @@ func TestIsBlockedIP_PrivateRanges(t *testing.T) {
 		{"192.168.0.1", "192.168.0.1", true},
 		{"192.168.255.255", "192.168.255.255", true},
 		{"169.254.0.1", "169.254.0.1", true},
-		{"169.254.169.254", "169.254.169.254", true},
+		{metadataIP, metadataIP, true},
 		{"8.8.8.8", "8.8.8.8", false},
 		{"1.1.1.1", "1.1.1.1", false},
-		{"93.184.216.34", "93.184.216.34", false},
+		{publicIP, publicIP, false},
 		{"::1", "::1", true},
 		{"fd00::1", "fd00::1", true},
 		{"fc00::1", "fc00::1", true},
@@ -72,12 +72,12 @@ func TestValidateTargetURL_Blocked(t *testing.T) {
 	t.Parallel()
 
 	blockedURLs := []string{
-		"http://127.0.0.1/hook",
+		loopbackHookURL,
 		"http://127.0.0.1:8080/hook",
 		"https://10.0.0.1/hook",
 		"http://192.168.1.1/webhook",
 		"http://172.16.0.1/api",
-		"http://169.254.169.254/latest/meta-data/",
+		metadataURL,
 		"http://[::1]/hook",
 		"http://[fc00::1]/hook",
 		"http://[fe80::1]/hook",
@@ -88,7 +88,7 @@ func TestValidateTargetURL_Blocked(t *testing.T) {
 		t.Run(u, func(t *testing.T) {
 			t.Parallel()
 
-			err := delivery.ValidateTargetURL(
+			err := delivery.NewTestGuard().ValidateTargetURL(
 				context.Background(), u,
 			)
 
@@ -112,7 +112,7 @@ func TestValidateTargetURL_Allowed(t *testing.T) {
 		t.Run(u, func(t *testing.T) {
 			t.Parallel()
 
-			err := delivery.ValidateTargetURL(
+			err := delivery.NewTestGuard().ValidateTargetURL(
 				context.Background(), u,
 			)
 
@@ -126,7 +126,7 @@ func TestValidateTargetURL_Allowed(t *testing.T) {
 func TestValidateTargetURL_InvalidScheme(t *testing.T) {
 	t.Parallel()
 
-	err := delivery.ValidateTargetURL(
+	err := delivery.NewTestGuard().ValidateTargetURL(
 		context.Background(), "ftp://example.com/hook",
 	)
 
@@ -140,7 +140,7 @@ func TestValidateTargetURL_InvalidScheme(t *testing.T) {
 func TestValidateTargetURL_EmptyHost(t *testing.T) {
 	t.Parallel()
 
-	err := delivery.ValidateTargetURL(
+	err := delivery.NewTestGuard().ValidateTargetURL(
 		context.Background(), "http:///path",
 	)
 
@@ -150,7 +150,7 @@ func TestValidateTargetURL_EmptyHost(t *testing.T) {
 func TestValidateTargetURL_InvalidURL(t *testing.T) {
 	t.Parallel()
 
-	err := delivery.ValidateTargetURL(
+	err := delivery.NewTestGuard().ValidateTargetURL(
 		context.Background(), "://invalid",
 	)