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/common.rs

@@ -216,7 +216,7 @@ pub fn settings_meta_plugin_types(
             if !found {
                 cmd.error(
                     ErrorKind::InvalidValue,
-                    format!("Unknown meta plugin type: {}", trimmed_name),
+                    format!("Unknown meta plugin type: {trimmed_name}"),
                 )
                 .exit();
             }
@@ -254,10 +254,7 @@ pub fn settings_compression_type(
     if compression_type_opt.is_err() {
         cmd.error(
             ErrorKind::InvalidValue,
-            format!(
-                "Invalid compression algorithm '{}'. Supported algorithms: lz4, gzip, xz, zstd",
-                compression_name
-            ),
+            format!("Invalid compression algorithm '{compression_name}'. Supported algorithms: lz4, gzip, xz, zstd"),
         )
         .exit();
     }