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.

sources/http_poll.v

@@ -0,0 +1,37 @@
+module sources
+
+pub fn transform_http_down(check HttpCheck, err string) Event {
+	mut message := ''
+	if check.template != '' {
+		vars := {
+			'url':    check.url
+			'error':  err
+			'status': ''
+		}
+		message = render_source_template(check.template, vars)
+	} else {
+		message = '❌ ${check.url} → ${err}'
+	}
+	return Event{
+		source:   'http_poll'
+		name:     check.url
+		topic:    check.topic
+		priority: check.priority
+		tags:     ['x']
+		message:  message
+		raw:      err
+	}
+}
+
+pub fn transform_http_up(check HttpCheck) Event {
+	message := '✅ ${check.url} is UP'
+	return Event{
+		source:   'http_poll'
+		name:     check.url
+		topic:    check.topic
+		priority: 1
+		tags:     ['white_check_mark']
+		message:  message
+		raw:      ''
+	}
+}