feat(server): add github_hmac and none webhook signing modes

Adds two new webhook trigger signing modes for external provider
compatibility:

- github_hmac: accepts X-Hub-Signature-256 header with
  HMAC-SHA256(secret, rawBody), no timestamp prefix. Compatible with
  GitHub, Sentry, and services following the same standard.
- none: no authentication; the 24-char hex publicId in the URL acts
  as the shared secret. For services that cannot add auth headers.

The replay window UI field is hidden when these modes are selected
since neither uses timestamp-based replay protection.

Closes #1892

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/src/__tests__/routines-service.test.ts

@@ -617,4 +617,72 @@ describeEmbeddedPostgres("routine service live-execution coalescing", () => {
     expect(run.status).toBe("issue_created");
     expect(run.linkedIssueId).toBeTruthy();
   });
+
+  it("accepts GitHub-style X-Hub-Signature-256 with github_hmac signing mode", async () => {
+    const { routine, svc } = await seedFixture();
+    const { trigger, secretMaterial } = await svc.createTrigger(
+      routine.id,
+      {
+        kind: "webhook",
+        signingMode: "github_hmac",
+      },
+      {},
+    );
+
+    const payload = { action: "opened", pull_request: { number: 1 } };
+    const rawBody = Buffer.from(JSON.stringify(payload));
+    const signature = `sha256=${createHmac("sha256", secretMaterial!.webhookSecret)
+      .update(rawBody)
+      .digest("hex")}`;
+
+    const run = await svc.firePublicTrigger(trigger.publicId!, {
+      hubSignatureHeader: signature,
+      rawBody,
+      payload,
+    });
+
+    expect(run.source).toBe("webhook");
+    expect(run.status).toBe("issue_created");
+  });
+
+  it("rejects invalid signature for github_hmac signing mode", async () => {
+    const { routine, svc } = await seedFixture();
+    const { trigger } = await svc.createTrigger(
+      routine.id,
+      {
+        kind: "webhook",
+        signingMode: "github_hmac",
+      },
+      {},
+    );
+
+    const rawBody = Buffer.from(JSON.stringify({ ok: true }));
+
+    await expect(
+      svc.firePublicTrigger(trigger.publicId!, {
+        hubSignatureHeader: "sha256=0000000000000000000000000000000000000000000000000000000000000000",
+        rawBody,
+        payload: { ok: true },
+      }),
+    ).rejects.toThrow();
+  });
+
+  it("accepts any request with none signing mode", async () => {
+    const { routine, svc } = await seedFixture();
+    const { trigger } = await svc.createTrigger(
+      routine.id,
+      {
+        kind: "webhook",
+        signingMode: "none",
+      },
+      {},
+    );
+
+    const run = await svc.firePublicTrigger(trigger.publicId!, {
+      payload: { event: "error.created" },
+    });
+
+    expect(run.source).toBe("webhook");
+    expect(run.status).toBe("issue_created");
+  });
 });