Atproto, serde deserialize & move strong_ref

This adds Serde deserialization. It also needs to add an error for
handling the failure on these deserializations.

I broke strong_ref into its own file because it was starting to grow a
lot.

Cargo.toml

@@ -7,7 +7,7 @@ async-trait = "0.1.88"
 atproto = { path = "./atproto" }
 db = { path = "./db" }
 router = { path = "./router" }
-serde = "1.0.219"
+serde = { version = "1.0.219", features = ["derive"] }
 serde_json = "1.0.140"
 sqlx = { version = "0.8.6", features = ["postgres", "runtime-tokio"] }
 thiserror = "2.0.12"