fix: resolve 3 CRITICAL + 5 MAJOR issues from Codex review

C1: Arc<Mutex<EventStore>> changed from tokio::sync to std::sync + spawn_blocking
C2: StateMachine::transition merged into single lock scope
C3: Transaction boundaries (BEGIN/COMMIT) on all composite writes
M4: retry_count no longer overwritten by update_task_status
M5: RetryPolicy::handle_failure now atomic (single lock + transaction)
M6: Per-task timeout_seconds used in SQL instead of global config
M7: Explicit Priority::order() method instead of relying on variant order
M8: dequeue_and_assign uses CAS-style WHERE status='created' for atomicity

src/core/models.rs

@@ -74,6 +74,29 @@ pub enum Priority {
     Urgent,
 }
 
+impl Priority {
+    /// Explicit priority ordering (lower = higher priority).
+    /// Not reliant on variant declaration order.
+    pub fn order(&self) -> u8 {
+        match self {
+            Self::Urgent => 0,
+            Self::High => 1,
+            Self::Normal => 2,
+            Self::Low => 3,
+        }
+    }
+
+    /// Serialize to the string stored in the DB.
+    pub fn as_str(&self) -> &'static str {
+        match self {
+            Self::Low => "low",
+            Self::Normal => "normal",
+            Self::High => "high",
+            Self::Urgent => "urgent",
+        }
+    }
+}
+
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct Task {
     pub task_id: String,