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.

config_test.v

@@ -0,0 +1,63 @@
+module main
+
+import sources
+
+fn test_validate_minimal_config() {
+	cfg := Config{
+		server:   ServerConfig{
+			url:    'https://ntfy.example.com'
+			listen: ':9090'
+		}
+		defaults: DefaultConfig{
+			priority: 3
+			tags:     ['loudspeaker']
+		}
+		sources:  SourcesConfig{
+			generic: [
+				sources.GenericSource{
+					name:         'test'
+					webhook_path: '/webhooks/test'
+					topic:        'test'
+				},
+			]
+		}
+		dedup:    DedupConfig{
+			enabled:     true
+			ttl_seconds: 300
+			rate_limit:  RateLimitConfig{
+				max_per_minute: 10
+				max_per_source: 30
+			}
+		}
+	}
+	validate_config(cfg) or { panic(err) }
+}
+
+fn test_validate_missing_url() {
+	cfg := Config{
+		server:   ServerConfig{
+			listen: ':9090'
+		}
+		defaults: DefaultConfig{
+			priority: 3
+		}
+	}
+	if _ := validate_config(cfg) {
+		panic('expected validation to fail')
+	}
+}
+
+fn test_validate_invalid_priority() {
+	cfg := Config{
+		server:   ServerConfig{
+			url:    'https://ntfy.example.com'
+			listen: ':9090'
+		}
+		defaults: DefaultConfig{
+			priority: 10
+		}
+	}
+	if _ := validate_config(cfg) {
+		panic('expected validation to fail')
+	}
+}