feat(grpc): implement MetadataService handlers

- Implement all 5 RPCs (Get, Update, Clear, Batch, Import)
- Add MetadataServiceImpl with database integration
- Add 10 comprehensive unit tests
- All 19 tests pass, full workspace compiles

Cargo.lock

@@ -616,6 +616,27 @@ dependencies = [
  "typenum",
 ]
 
+[[package]]
+name = "csv"
+version = "1.4.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "52cd9d68cf7efc6ddfaaee42e7288d3a99d613d4b50f76ce9827ae0c6e14f938"
+dependencies = [
+ "csv-core",
+ "itoa",
+ "ryu",
+ "serde_core",
+]
+
+[[package]]
+name = "csv-core"
+version = "0.1.13"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "704a3c26996a80471189265814dbc2c257598b96b8a7feae2d31ace646bb9782"
+dependencies = [
+ "memchr",
+]
+
 [[package]]
 name = "dashmap"
 version = "5.5.3"
@@ -2020,8 +2041,10 @@ name = "musicfs-grpc"
 version = "0.1.0"
 dependencies = [
  "chrono",
+ "csv",
  "hex",
  "hmac",
+ "musicfs-cache",
  "musicfs-core",
  "musicfs-search",
  "prost",