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/compression_engine/mod.rs

@@ -1,4 +1,4 @@
-use anyhow::{Result, anyhow};
+use anyhow::{anyhow, Result};
 use std::io;
 use std::io::{Read, Write};
 use std::path::PathBuf;
@@ -73,14 +73,14 @@ pub trait CompressionEngine: Send + Sync {
     ///
     /// # Returns
     ///
-    /// * `Result<Box<dyn Read>>` - A boxed reader that decompresses the file on read,
+    /// * `Result<Box<dyn Read + Send>>` - A boxed reader that decompresses the file on read,
     ///   or an error if the file cannot be opened or is invalid.
     ///
     /// # Errors
     ///
     /// Returns an error if the file does not exist, is not a valid compressed file,
     /// or if decompression fails.
-    fn open(&self, file_path: PathBuf) -> Result<Box<dyn Read>>;
+    fn open(&self, file_path: PathBuf) -> Result<Box<dyn Read + Send>>;
 
     /// Creates a new compressed file for writing.
     ///