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/tests/server/auth_tests.rs

@@ -1,4 +1,4 @@
-#[cfg(test)]
+#[cfg(all(test, feature = "server"))]
 mod tests {
     use crate::modes::server::common::check_auth;
     use axum::http::{HeaderMap, HeaderValue};