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/meta_plugin/magic_file.rs

@@ -54,11 +54,11 @@ impl MagicFileMetaPluginImpl {
         if let Some(cookie) = &self.cookie {
             cookie
                 .set_flags(flags)
-                .map_err(|e| io::Error::other(format!("Failed to set magic flags: {}", e)))?;
+                .map_err(|e| io::Error::other(format!("Failed to set magic flags: {e}")))?;
 
             let result = cookie
                 .buffer(&self.buffer)
-                .map_err(|e| io::Error::other(format!("Failed to analyze buffer: {}", e)))?;
+                .map_err(|e| io::Error::other(format!("Failed to analyze buffer: {e}")))?;
 
             // Clean up the result - remove extra whitespace
             let trimmed = result.trim().to_string();
@@ -109,7 +109,7 @@ impl MetaPlugin for MagicFileMetaPluginImpl {
         let cookie = match Cookie::open(CookieFlags::default()) {
             Ok(cookie) => cookie,
             Err(e) => {
-                debug!("META: MagicFile plugin: failed to create cookie: {}", e);
+                debug!("META: MagicFile plugin: failed to create cookie: {e}");
                 return MetaPluginResponse {
                     metadata: Vec::new(),
                     is_finalized: true,
@@ -118,10 +118,7 @@ impl MetaPlugin for MagicFileMetaPluginImpl {
         };
 
         if let Err(e) = cookie.load(&[] as &[&Path]) {
-            debug!(
-                "META: MagicFile plugin: failed to load magic database: {}",
-                e
-            );
+            debug!("META: MagicFile plugin: failed to load magic database: {e}");
             return MetaPluginResponse {
                 metadata: Vec::new(),
                 is_finalized: true,