server/json-rpc: Replace jsonrpsee with axum

Migrate the JSON-RPC admin server from jsonrpsee to axum/tower, which
are already used elsewhere in the codebase (server/graphman). This
removes the jsonrpsee dependency while maintaining full protocol
compatibility.

Changes:
- Add jsonrpc.rs: JSON-RPC 2.0 types (request, response, error, ID)
- Add error.rs: Error code mapping from SubgraphRegistrarError
- Add handlers.rs: Request dispatch and method handlers
- Add server.rs: Axum-based HTTP server with graceful shutdown
- Update lib.rs: Module structure with public exports
- Update Cargo.toml: Replace jsonrpsee with axum, serde_json, thiserror

Author: lutter

Cargo.lock

@@ -4,6 +4,10 @@ version.workspace = true
 edition.workspace = true
 
 [dependencies]
+axum = { workspace = true }
 graph = { path = "../../graph" }
-jsonrpsee = { version = "0.15.1", features = ["http-server"] }
 serde = { workspace = true }
+serde_json = { workspace = true }
+slog = { workspace = true }
+thiserror = { workspace = true }
+tokio = { workspace = true }