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/server/api/common.rs

@@ -1,9 +1,9 @@
 use axum::{
-    http::{header, StatusCode},
+    http::{StatusCode, header},
     response::Response,
 };
-use serde::Serialize;
 use log;
+use serde::Serialize;
 
 pub struct ResponseBuilder;
 
@@ -13,7 +13,7 @@ impl ResponseBuilder {
             log::warn!("Failed to serialize response: {}", e);
             StatusCode::INTERNAL_SERVER_ERROR
         })?;
-        
+
         Response::builder()
             .header(header::CONTENT_TYPE, "application/json")
             .header(header::CONTENT_LENGTH, json.len().to_string())
@@ -23,7 +23,7 @@ impl ResponseBuilder {
                 StatusCode::INTERNAL_SERVER_ERROR
             })
     }
-    
+
     pub fn binary(content: &[u8], mime_type: &str) -> Result<Response, StatusCode> {
         Response::builder()
             .header(header::CONTENT_TYPE, mime_type)