feat: inject build-time variables via Gradle processResources

Replace hardcoded AppProperties values with Gradle ${} placeholders,
allowing version/channel/vendor to be configured via gradle.properties
or -P flags at build time.

Also refactor webhook configuration to flatten the properties hierarchy
by removing the intermediate WebhookProperties wrapper.

build.gradle.kts

@@ -5,6 +5,8 @@ plugins {
 }
 
 val artefactVersion: String by project
+val buildChannel: String by project
+val vendor: String by project
 
 group = "com.onixbyte.helix"
 version = artefactVersion
@@ -61,6 +63,16 @@ dependencies {
     testRuntimeOnly(libs.junit.launcher)
 }
 
+tasks.processResources {
+    filesMatching("application.yaml") {
+        expand(
+            "appVersion" to artefactVersion,
+            "channel" to buildChannel,
+            "vendor" to vendor
+        )
+    }
+}
+
 tasks.test {
     useJUnitPlatform()
 }