Add MongoDB reachability validation to IPL sequence

- Add rec_services config section to beds.toml and test fixture
- Add RecNodeConfig struct; export from config module
- Add mongo::validate() and validate_all() — TCP ping per configured REC node
- Wire mongo::validate_all() into ipl() with env-aware error handling
- Add mongodb crate dependency (sync feature)
- Add unit tests for mongo validate error paths (closed port, bad address)
- Update README status table and project structure

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

config/beds.toml

@@ -30,4 +30,12 @@ rpi = 50
 [broker_services.app_server.instances]
 r_broker = 2
 w_broker = 2
-m_broker = 0
+m_broker = 0
+
+[rec_services.app_server]
+host     = "localhost"
+port     = 27017
+user     = "beds"
+pass     = "changeme"
+database = "beds_app"
+use_ssl  = false