feat(alerting): HmacSigner for webhook signature

HmacSHA256 signer returning sha256=<lowercase-hex>. 5 unit tests covering
known vector, prefix, hex casing, and different secrets/bodies.

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

cameleer-server-app/src/test/java/com/cameleer/server/app/alerting/notify/HmacSignerTest.java

@@ -0,0 +1,55 @@
+package com.cameleer.server.app.alerting.notify;
+
+import org.junit.jupiter.api.Test;
+
+import java.nio.charset.StandardCharsets;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+class HmacSignerTest {
+
+    private final HmacSigner signer = new HmacSigner();
+
+    /**
+     * Pre-computed:
+     *   secret = "test-secret-key"
+     *   body   = "hello world"
+     *   result = sha256=b3df71b4790eb32b24c2f0bbb20f215d82b0da5e921caa880c74acfc97cf7e5b
+     *
+     * Verified with: python3 -c "import hmac,hashlib; print('sha256='+hmac.new(b'test-secret-key',b'hello world',hashlib.sha256).hexdigest())"
+     */
+    @Test
+    void knownVector() {
+        String result = signer.sign("test-secret-key", "hello world".getBytes(StandardCharsets.UTF_8));
+        assertThat(result).isEqualTo("sha256=b3df71b4790eb32b24c2f0bbb20f215d82b0da5e921caa880c74acfc97cf7e5b");
+    }
+
+    @Test
+    void outputStartsWithSha256Prefix() {
+        String result = signer.sign("any-secret", "body".getBytes(StandardCharsets.UTF_8));
+        assertThat(result).startsWith("sha256=");
+    }
+
+    @Test
+    void outputIsLowercaseHex() {
+        String result = signer.sign("key", "data".getBytes(StandardCharsets.UTF_8));
+        // After "sha256=" every char must be a lowercase hex digit
+        String hex = result.substring("sha256=".length());
+        assertThat(hex).matches("[0-9a-f]{64}");
+    }
+
+    @Test
+    void differentSecretsProduceDifferentSignatures() {
+        byte[] body = "payload".getBytes(StandardCharsets.UTF_8);
+        String sig1 = signer.sign("secret-a", body);
+        String sig2 = signer.sign("secret-b", body);
+        assertThat(sig1).isNotEqualTo(sig2);
+    }
+
+    @Test
+    void differentBodiesProduceDifferentSignatures() {
+        String sig1 = signer.sign("secret", "body1".getBytes(StandardCharsets.UTF_8));
+        String sig2 = signer.sign("secret", "body2".getBytes(StandardCharsets.UTF_8));
+        assertThat(sig1).isNotEqualTo(sig2);
+    }
+}