Add machine deletion API with uninstall command support

- Add AdminCommand message to protobuf (uninstall, restart, update)
- Add DELETE /api/machines/:agent_id endpoint with options:
  - ?uninstall=true - send uninstall command to online agent
  - ?export=true - return session history before deletion
- Add GET /api/machines/:agent_id/history endpoint for history export
- Add GET /api/machines endpoint to list all machines
- Handle AdminCommand in agent session handler
- Handle ADMIN_UNINSTALL error in agent main loop to trigger uninstall

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

server/src/db/sessions.rs

@@ -96,3 +96,16 @@ pub async fn get_recent_sessions(
     .fetch_all(pool)
     .await
 }
+
+/// Get all sessions for a machine (for history export)
+pub async fn get_sessions_for_machine(
+    pool: &PgPool,
+    machine_id: Uuid,
+) -> Result<Vec<DbSession>, sqlx::Error> {
+    sqlx::query_as::<_, DbSession>(
+        "SELECT * FROM connect_sessions WHERE machine_id = $1 ORDER BY started_at DESC"
+    )
+    .bind(machine_id)
+    .fetch_all(pool)
+    .await
+}