add http-json, rss, and media data source adapters

http-json: generic HTTP+JSON polling adapter, converts serde_json to domain Value. 4 tests.
rss: XML RSS feed parser, extracts items into Value array. 1 test.
media: Navidrome/Subsonic getNowPlaying adapter. 2 tests with fake server.

crates/adapters/http-json/Cargo.toml

@@ -0,0 +1,13 @@
+[package]
+name = "http-json"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
+domain.workspace = true
+reqwest.workspace = true
+serde_json.workspace = true
+
+[dev-dependencies]
+tokio.workspace = true
+axum.workspace = true