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

crates/musicfs-grpc/Cargo.toml

@@ -4,6 +4,7 @@ version.workspace = true
 edition.workspace = true
 
 [dependencies]
+musicfs-cache = { path = "../musicfs-cache" }
 musicfs-search = { path = "../musicfs-search" }
 musicfs-core = { path = "../musicfs-core" }
 tonic.workspace = true
@@ -15,6 +16,7 @@ thiserror.workspace = true
 serde.workspace = true
 serde_json.workspace = true
 chrono.workspace = true
+csv = "1.3"
 reqwest = { version = "0.11", features = ["json"] }
 hmac = "0.12"
 sha2 = "0.10"