🔒️(backend) validate Room configuration with Pydantic schema

Room.configuration accepted arbitrary JSON without validation, allowing unsafe
or malformed payloads to be stored and creating a security risk. Define a
Pydantic schema to enforce structure and constraints, and add validation
at the serializer level to reject invalid inputs.

CHANGELOG.md

@@ -8,6 +8,10 @@ and this project adheres to
 
 ## [Unreleased]
 
+### Added
+
+- 🔒️(backend) add validation of Room.configuration
+
 ## [1.15.0] - 2026-04-30
 
 ### Added