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/proxy/surfaces/mod.rs

@@ -0,0 +1,32 @@
+use std::collections::HashMap;
+use std::sync::Arc;
+
+use parking_lot::Mutex;
+use serde_json::Value;
+
+#[derive(Debug, Clone, Default)]
+pub struct ProxySurfaceState {
+    sessions: Arc<Mutex<HashMap<String, SurfaceSession>>>,
+}
+
+#[derive(Debug, Clone)]
+pub struct SurfaceSession {
+    pub id: String,
+    pub model: String,
+    pub channel_id: Option<i64>,
+    pub metadata: Value,
+}
+
+impl ProxySurfaceState {
+    pub fn upsert_session(&self, session: SurfaceSession) {
+        self.sessions.lock().insert(session.id.clone(), session);
+    }
+
+    pub fn get_session(&self, id: &str) -> Option<SurfaceSession> {
+        self.sessions.lock().get(id).cloned()
+    }
+
+    pub fn remove_session(&self, id: &str) -> Option<SurfaceSession> {
+        self.sessions.lock().remove(id)
+    }
+}