refactor: purge Node.js, Rust is the one true backend

- Moved boosapi-rs/* to repo root
- Deleted all Node.js source (src/server/, src/web/, scripts/)
- Deleted node_modules, package.json, tsconfig, vite.config
- Updated .gitignore and AGENTS.md for Rust-only stack
- Fixed static file path for frontend dist/web/
- Dockerfile stays the same (already Rust-only)
- Kept dist/web/ for Rust static serving

src/transformers/shared/mod.rs

@@ -0,0 +1,22 @@
+use serde_json::Value;
+
+pub fn string_field<'a>(body: &'a Value, key: &str) -> Option<&'a str> {
+    body.get(key).and_then(Value::as_str)
+}
+
+pub fn bool_field(body: &Value, key: &str) -> Option<bool> {
+    body.get(key).and_then(Value::as_bool)
+}
+
+pub fn array_field<'a>(body: &'a Value, key: &str) -> Option<&'a Vec<Value>> {
+    body.get(key).and_then(Value::as_array)
+}
+
+pub fn merge_json_object(base: &mut Value, extra: Value) {
+    let (Some(base), Some(extra)) = (base.as_object_mut(), extra.as_object()) else {
+        return;
+    };
+    for (key, value) in extra {
+        base.insert(key.clone(), value.clone());
+    }
+}