feat(webhook): HMAC-SHA256 signature verification on inbound webhooks

Adds an opt-in inbound HMAC scheme so a leaked URL alone is not enough
to forge deploy/sync requests — the caller must also know a separate
signing secret. Header format is X-Hub-Signature-256, matching the
Gitea/GitHub/GitLab convention so existing CI integrations work without
custom code.

Behaviour:
- per-project / per-site signing_secret is independent of the URL secret
- require_signature flag does a hard 401 on missing/invalid signatures
- even when require_signature is off, an *invalid* submitted signature
  returns 401 — surfaces CI misconfiguration instead of silently passing
- comparison uses subtle/hmac.Equal (constant time)

Backend:
- store: webhook_signing_secret + webhook_require_signature columns on
  projects + static_sites; scanProject helper, scan helpers updated; new
  Set* helpers for both fields
- webhook/handler: verifyHMAC helper, body read once, integrated into
  both project and site handlers
- api: per-entity signing-secret rotate / disable / require-toggle
  endpoints under /api/{projects,sites}/{id}/webhook/...

Frontend:
- WebhookPanel gains optional signing handlers (no breaking change for
  existing callers; signing UI hides when handlers aren't wired)
- one-shot reveal of the issued secret with copy + dismiss
- ToggleSwitch for require-signature, disabled until a secret is issued
- en/ru i18n strings

Tests:
- HMACRequiredAndValid (200 + deploy fires)
- HMACRequiredButMissing (401, no deploy)
- HMACPresentButWrong (401 even when require_signature=false)
- HMACOptionalUnsignedAccepted (200 when neither configured)

internal/webhook/handler_test.go

@@ -2,6 +2,9 @@ package webhook_test
 
 import (
 	"context"
+	"crypto/hmac"
+	"crypto/sha256"
+	"encoding/hex"
 	"encoding/json"
 	"io"
 	"net/http"
@@ -16,6 +19,29 @@ import (
 	"github.com/alexei/tinyforge/internal/webhook"
 )
 
+// signBody computes the HMAC-SHA256 hex digest used by the X-Hub-Signature-256 header.
+func signBody(secret, body string) string {
+	mac := hmac.New(sha256.New, []byte(secret))
+	mac.Write([]byte(body))
+	return "sha256=" + hex.EncodeToString(mac.Sum(nil))
+}
+
+// doJSONSigned mirrors doJSON but adds the X-Hub-Signature-256 header.
+func doJSONSigned(t *testing.T, r chi.Router, method, path, body, signingSecret string) (*http.Response, string) {
+	t.Helper()
+	req := httptest.NewRequest(method, path, strings.NewReader(body))
+	req.Header.Set("Content-Type", "application/json")
+	if signingSecret != "" {
+		req.Header.Set("X-Hub-Signature-256", signBody(signingSecret, body))
+	}
+	w := httptest.NewRecorder()
+	r.ServeHTTP(w, req)
+	resp := w.Result()
+	b, _ := io.ReadAll(resp.Body)
+	resp.Body.Close()
+	return resp, string(b)
+}
+
 // fakeDeployer records the last trigger for assertion.
 type fakeDeployer struct {
 	mu       sync.Mutex
@@ -309,3 +335,123 @@ func TestSiteWebhook_PushSkippedForNonMatchingBranch(t *testing.T) {
 		t.Errorf("non-matching branch must not trigger sync; got %d calls", ft.calls)
 	}
 }
+
+// HMAC enforcement scenarios.
+
+func TestProjectWebhook_HMACRequiredAndValid(t *testing.T) {
+	t.Parallel()
+	st := newStore(t)
+	p, _ := st.CreateProject(store.Project{
+		Name: "app", Image: "alexei/app", Env: "{}", Volumes: "{}",
+	})
+	if _, err := st.CreateStage(store.Stage{
+		ProjectID: p.ID, Name: "dev", TagPattern: "*", AutoDeploy: true, MaxInstances: 1,
+	}); err != nil {
+		t.Fatal(err)
+	}
+	const sig = "deadbeef-signing-secret-1234567890abcdef"
+	if err := st.SetProjectWebhookSigningSecret(p.ID, sig); err != nil {
+		t.Fatal(err)
+	}
+	if err := st.SetProjectWebhookRequireSignature(p.ID, true); err != nil {
+		t.Fatal(err)
+	}
+
+	dep := &fakeDeployer{}
+	h := webhook.NewHandler(st, dep, nil)
+	r := newRouter(t, h)
+
+	body := `{"image":"alexei/app:dev-abc"}`
+	resp, msg := doJSONSigned(t, r, http.MethodPost, "/api/webhook/"+p.WebhookSecret, body, sig)
+	if resp.StatusCode != http.StatusOK {
+		t.Fatalf("expected 200 with valid sig, got %d: %s", resp.StatusCode, msg)
+	}
+	if dep.calls != 1 {
+		t.Errorf("valid signed deploy should fire once, got %d", dep.calls)
+	}
+}
+
+func TestProjectWebhook_HMACRequiredButMissing(t *testing.T) {
+	t.Parallel()
+	st := newStore(t)
+	p, _ := st.CreateProject(store.Project{
+		Name: "app", Image: "alexei/app", Env: "{}", Volumes: "{}",
+	})
+	if _, err := st.CreateStage(store.Stage{
+		ProjectID: p.ID, Name: "dev", TagPattern: "*", AutoDeploy: true, MaxInstances: 1,
+	}); err != nil {
+		t.Fatal(err)
+	}
+	if err := st.SetProjectWebhookSigningSecret(p.ID, "abc-signing-secret-12345678901234567890"); err != nil {
+		t.Fatal(err)
+	}
+	if err := st.SetProjectWebhookRequireSignature(p.ID, true); err != nil {
+		t.Fatal(err)
+	}
+
+	dep := &fakeDeployer{}
+	h := webhook.NewHandler(st, dep, nil)
+	r := newRouter(t, h)
+
+	resp, _ := doJSON(t, r, http.MethodPost, "/api/webhook/"+p.WebhookSecret, `{"image":"alexei/app:dev-abc"}`)
+	if resp.StatusCode != http.StatusUnauthorized {
+		t.Fatalf("missing signature must return 401 when required, got %d", resp.StatusCode)
+	}
+	if dep.calls != 0 {
+		t.Errorf("deploy must not fire when required signature is missing")
+	}
+}
+
+func TestProjectWebhook_HMACPresentButWrong(t *testing.T) {
+	t.Parallel()
+	st := newStore(t)
+	p, _ := st.CreateProject(store.Project{
+		Name: "app", Image: "alexei/app", Env: "{}", Volumes: "{}",
+	})
+	if _, err := st.CreateStage(store.Stage{
+		ProjectID: p.ID, Name: "dev", TagPattern: "*", AutoDeploy: true, MaxInstances: 1,
+	}); err != nil {
+		t.Fatal(err)
+	}
+	if err := st.SetProjectWebhookSigningSecret(p.ID, "real-signing-secret-1234567890abcdef"); err != nil {
+		t.Fatal(err)
+	}
+	// Note: require_signature stays false — but a wrong sig must still 401.
+
+	dep := &fakeDeployer{}
+	h := webhook.NewHandler(st, dep, nil)
+	r := newRouter(t, h)
+
+	resp, _ := doJSONSigned(t, r, http.MethodPost, "/api/webhook/"+p.WebhookSecret,
+		`{"image":"alexei/app:dev-abc"}`, "wrong-secret-xxxxxxxxxxxxxxxxxxxxxxxxxxxx")
+	if resp.StatusCode != http.StatusUnauthorized {
+		t.Fatalf("wrong signature must 401, got %d", resp.StatusCode)
+	}
+	if dep.calls != 0 {
+		t.Errorf("deploy must not fire on wrong signature")
+	}
+}
+
+func TestProjectWebhook_HMACOptionalUnsignedAccepted(t *testing.T) {
+	// require_signature=false AND signing_secret="": unsigned requests pass.
+	t.Parallel()
+	st := newStore(t)
+	p, _ := st.CreateProject(store.Project{
+		Name: "app", Image: "alexei/app", Env: "{}", Volumes: "{}",
+	})
+	if _, err := st.CreateStage(store.Stage{
+		ProjectID: p.ID, Name: "dev", TagPattern: "*", AutoDeploy: true, MaxInstances: 1,
+	}); err != nil {
+		t.Fatal(err)
+	}
+	dep := &fakeDeployer{}
+	h := webhook.NewHandler(st, dep, nil)
+	r := newRouter(t, h)
+	resp, _ := doJSON(t, r, http.MethodPost, "/api/webhook/"+p.WebhookSecret, `{"image":"alexei/app:dev-x"}`)
+	if resp.StatusCode != http.StatusOK {
+		t.Fatalf("unsigned + unconfigured should pass, got %d", resp.StatusCode)
+	}
+	if dep.calls != 1 {
+		t.Errorf("expected 1 deploy, got %d", dep.calls)
+	}
+}