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/digest.rs

@@ -42,15 +42,15 @@ impl Hasher {
         match self {
             Hasher::Sha256(hasher) => {
                 let result = std::mem::replace(hasher, Sha256::new()).finalize_reset();
-                format!("{:x}", result)
+                format!("{result:x}")
             }
             Hasher::Md5(hasher) => {
                 let result = hasher.clone().compute();
-                format!("{:x}", result)
+                format!("{result:x}")
             }
             Hasher::Sha512(hasher) => {
                 let result = std::mem::replace(hasher, Sha512::new()).finalize_reset();
-                format!("{:x}", result)
+                format!("{result:x}")
             }
         }
     }
@@ -119,7 +119,7 @@ impl DigestMetaPlugin {
         // Only the selected method's output should be enabled, others should be None
         let all_outputs = vec!["digest_md5", "digest_sha256", "digest_sha512"];
         for output_name in &all_outputs {
-            if output_name == &format!("digest_{}", method) {
+            if output_name == &format!("digest_{method}") {
                 base.outputs.insert(
                     output_name.to_string(),
                     serde_yaml::Value::String(output_name.to_string()),