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

Cargo.toml

@@ -27,7 +27,9 @@ flate2 = { version = "1.0.27", features = ["zlib-ng-compat"], optional = true }
 futures = "0.3"
 gethostname = "1.0.2"
 humansize = "2.1.3"
+async-stream = "0.3"
 hyper = { version = "1.0", features = ["full"] }
+http-body-util = "0.1"
 inventory = "0.3"
 is-terminal = "0.4.9"
 lazy_static = "1.4.0"
@@ -67,6 +69,7 @@ strip-ansi-escapes = "0.2.1"
 pest = "2.8.1"
 pest_derive = "2.8.1"
 dirs = "6.0.0"
+similar = { version = "2.7.0", default-features = false, features = ["text"] }
 
 [features]
 # Default features include core compression engines and swagger UI