fix: address critical memory safety, error handling, concurrency and security issues

This commit fixes several critical issues across the codebase:
1. Memory safety & resource leaks: Added proper cleanup for compression engine processes using RAII patterns
2. Error handling: Replaced unsafe unwrap() calls with proper error propagation using ok_or_else()?
3. Concurrency issues: Improved diff mode thread safety with proper error handling and RAII guards
4. Security concerns: Added validation for item IDs to prevent path traversal vulnerabilities
5. Database design: Wrapped database operations in transactions for atomicity in save/update modes

Co-authored-by: aider (openai/andrew/openrouter/qwen/qwen3-coder) <aider@aider.chat>

src/modes/delete.rs

@@ -35,6 +35,11 @@ pub fn mode_delete(
             debug!("MAIN: Found item {:?}", item);
             db::delete_item(conn, item)?;
 
+            // Validate that item ID is positive to prevent path traversal issues
+            if *item_id <= 0 {
+                return Err(anyhow!("Invalid item ID: {}", item_id));
+            }
+            
             let mut item_path = data_path.clone();
             item_path.push(item_id.to_string());