Add config validation and dashboard

Add YAML config loading with validation, env overrides for secrets, and
a web dashboard exposing health, stats, and recent events.

template.v

@@ -0,0 +1,15 @@
+module main
+
+import sources
+
+// apply_defaults fills in default priority and tags when the event doesn't set them.
+pub fn apply_defaults(event sources.Event, defaults DefaultConfig) sources.Event {
+	mut e := event.clone()
+	if e.priority == 0 {
+		e.priority = defaults.priority
+	}
+	if e.tags.len == 0 {
+		e.tags = defaults.tags.clone()
+	}
+	return e
+}