Adopt rustfmt and clean up clippy warnings

Formats the whole backend to rustfmt defaults and fixes all 23
outstanding clippy warnings (collapsible ifs, clone-to-slice-ref,
type-complexity, single-arm match, unnecessary is_none check), then
wires cargo fmt --check and -D warnings into make lint so both stay
enforced going forward.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TeyhRfMYjMditek6geDRoQ

src/engine/data_value.rs

@@ -39,7 +39,8 @@ pub fn extract_json_field(text: &str, field_path: &str) -> Option<String> {
 }
 
 pub fn flatten_json_object(text: &str) -> Vec<(String, String)> {
-    let Ok(serde_json::Value::Object(root)) = serde_json::from_str::<serde_json::Value>(text) else {
+    let Ok(serde_json::Value::Object(root)) = serde_json::from_str::<serde_json::Value>(text)
+    else {
         return Vec::new();
     };
     let mut results = Vec::new();
@@ -53,7 +54,11 @@ fn flatten_json_object_into(
     results: &mut Vec<(String, String)>,
 ) {
     for (key, value) in object {
-        let path = if prefix.is_empty() { key.clone() } else { format!("{prefix}.{key}") };
+        let path = if prefix.is_empty() {
+            key.clone()
+        } else {
+            format!("{prefix}.{key}")
+        };
         match value {
             serde_json::Value::String(s) => results.push((path, s.clone())),
             serde_json::Value::Number(n) => results.push((path, n.to_string())),
@@ -78,7 +83,9 @@ pub enum MetricPrimitiveKind {
 #[allow(dead_code)]
 #[derive(Debug, Clone, Serialize)]
 pub enum DataValue {
-    LogLiteral { text: String },
+    LogLiteral {
+        text: String,
+    },
     ParamDeclaration(ParamDeclaration),
     MetricDeclaration {
         message_match_regex: String,