feat: add build version with timestamp

- Add build.rs to generate BUILD_VERSION at compile time
- Update CLI to show full version: '0.1.0 (build: 2026-03-31 11:21:37)'
- Update health endpoints to return build version
- Add chrono as build dependency

Cargo.toml

@@ -110,3 +110,6 @@ path = "src/bin/migrate_chinese_text.rs"
 [[bin]]
 name = "test_bm25_simple"
 path = "src/bin/test_bm25_simple.rs"
+
+[build-dependencies]
+chrono = "0.4"