fix: correct critical bugs and improve pipe streaming performance

Critical bug fixes:
- save_item now returns real Item from database, not a hardcoded fake
- AsyncDataService::save() reuses self.sync_service instead of creating redundant instance
- GenerateStatus trait signature mismatch fixed (CLI/API decoupling)

Performance improvements (pipe path untouched):
- CompressionEngine::open() returns Box<dyn Read + Send> enabling true streaming
- mode_get eliminates triple full-file read (was sampling then re-reading entire file)
- FilteringReader adds fast-path bypass when no filters, pre-allocates temp buffer
- text.rs meta plugin processes &[u8] slice directly, eliminates data.to_vec() clone

API correctness:
- Tag parse errors now return 400 instead of being silently discarded
- compute_diff uses similar crate (LCS-based) instead of naive positional comparison

Cleanup:
- Modernize string formatting (format!({x})) across codebase
- Remove redundant DB query in get mode
- Derive Debug/ToSchema on public types
- Delete placeholder test files with no real assertions
- Extract parse_comma_tags utility function

src/modes/diff.rs

@@ -8,11 +8,7 @@ use anyhow::{Context, Result};
 use clap::Command;
 use log::debug;
 
-fn validate_diff_args(
-    _cmd: &mut Command,
-    ids: &Vec<i64>,
-    tags: &Vec<String>,
-) -> anyhow::Result<()> {
+fn validate_diff_args(_cmd: &mut Command, ids: &[i64], tags: &[String]) -> anyhow::Result<()> {
     if !tags.is_empty() {
         return Err(anyhow::anyhow!(
             "Tags are not supported with --diff. Please provide exactly two IDs."
@@ -137,9 +133,46 @@ pub fn mode_diff(
 
     let (path_a, path_b) = setup_diff_paths_and_compression(&item_service, &item_a, &item_b)?;
 
-    // TODO: Implement actual diff logic here
-    // For now, just print paths or something to make it compile
-    println!("Diff between {:?} and {:?}", path_a, path_b);
+    run_external_diff(&path_a, &path_b)?;
 
     Ok(())
 }
+
+/// Runs external diff command to compare two files.
+///
+/// Uses the system's `diff` command to generate a unified diff output.
+/// Returns an error if the diff command is not found.
+///
+/// # Arguments
+///
+/// * `path_a` - Path to the first file.
+/// * `path_b` - Path to the second file.
+///
+/// # Returns
+///
+/// * `Result<()>` - Success or error.
+fn run_external_diff(path_a: &std::path::Path, path_b: &std::path::Path) -> anyhow::Result<()> {
+    if which::which_global("diff").is_err() {
+        return Err(anyhow::anyhow!(
+            "diff command not found. Please install diffutils."
+        ));
+    }
+
+    let mut child = std::process::Command::new("diff")
+        .arg("-u")
+        .arg(path_a)
+        .arg(path_b)
+        .stdout(std::process::Stdio::inherit())
+        .stderr(std::process::Stdio::inherit())
+        .spawn()
+        .context("Failed to spawn diff command")?;
+
+    let status = child.wait().context("Failed to wait for diff command")?;
+
+    // diff returns 0 if files are identical, 1 if different, 2 on error
+    if status.code() == Some(2) {
+        Err(anyhow::anyhow!("diff command failed with an error"))
+    } else {
+        Ok(())
+    }
+}