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/mod.rs

@@ -251,14 +251,14 @@ pub fn process_metadata_outputs(
     if let Some(mapping) = outputs.get(internal_name) {
         // Check for null to disable the output
         if mapping.is_null() {
-            debug!("META: Skipping disabled output (null): {}", internal_name);
+            debug!("META: Skipping disabled output (null): {internal_name}");
             return None;
         }
         // Check for boolean false to disable the output
         if let Some(false_val) = mapping.as_bool()
             && !false_val
         {
-            debug!("META: Skipping disabled output: {}", internal_name);
+            debug!("META: Skipping disabled output: {internal_name}");
             return None;
         }
         if let Some(custom_name) = mapping.as_str() {
@@ -279,8 +279,7 @@ pub fn process_metadata_outputs(
                 }
             };
             debug!(
-                "META: Processing metadata: internal_name={}, custom_name={}, value={}",
-                internal_name, custom_name, value_str
+                "META: Processing metadata: internal_name={internal_name}, custom_name={custom_name}, value={value_str}"
             );
             return Some(MetaData {
                 name: custom_name.to_string(),
@@ -307,10 +306,7 @@ pub fn process_metadata_outputs(
     };
 
     // Default: use internal name as output name
-    debug!(
-        "META: Processing metadata: name={}, value={}",
-        internal_name, value_str
-    );
+    debug!("META: Processing metadata: name={internal_name}, value={value_str}");
     Some(MetaData {
         name: internal_name.to_string(),
         value: value_str,
@@ -507,5 +503,5 @@ pub fn get_meta_plugin(
     }
 
     // Fallback for unknown plugins
-    panic!("Meta plugin {:?} not registered", meta_plugin_type);
+    panic!("Meta plugin {meta_plugin_type:?} not registered");
 }