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.lock

@@ -130,6 +130,28 @@ version = "0.5.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "7d902e3d592a523def97af8f317b08ce16b7ab854c1985a0c671e6f15cebc236"
 
+[[package]]
+name = "async-stream"
+version = "0.3.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0b5a71a6f37880a80d1d7f19efd781e4b5de42c88f0722cc13bcb6cc2cfe8476"
+dependencies = [
+ "async-stream-impl",
+ "futures-core",
+ "pin-project-lite",
+]
+
+[[package]]
+name = "async-stream-impl"
+version = "0.3.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c7c24de15d275a1ecfd47a380fb4d5ec9bfe0933f309ed5e705b775596a3574d"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn 2.0.105",
+]
+
 [[package]]
 name = "async-trait"
 version = "0.1.89"
@@ -1377,6 +1399,7 @@ name = "keep"
 version = "0.1.0"
 dependencies = [
  "anyhow",
+ "async-stream",
  "axum",
  "base64 0.22.1",
  "chrono",
@@ -1392,6 +1415,7 @@ dependencies = [
  "flate2",
  "futures",
  "gethostname",
+ "http-body-util",
  "humansize",
  "hyper",
  "inventory",
@@ -1418,6 +1442,7 @@ dependencies = [
  "serde_json",
  "serde_yaml",
  "sha2 0.10.9",
+ "similar",
  "smart-default",
  "stderrlog",
  "strip-ansi-escapes",
@@ -2325,6 +2350,12 @@ version = "0.3.7"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "d66dc143e6b11c1eddc06d5c423cfc97062865baf299914ab64caa38182078fe"
 
+[[package]]
+name = "similar"
+version = "2.7.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "bbbb5d9659141646ae647b42fe094daf6c6192d1620870b449d9557f748b2daa"
+
 [[package]]
 name = "slab"
 version = "0.4.11"