feat: add agent that is able to receive messages from UI

Inside the `/message` enpoint we put a message onto the internal event
queue, which gets read by TestAgent. This agent, in turn, logs the
message (temporary behaviour until we implement RI integration).

The name TestAgent is of course temporary, this is just for exploratory
purposes.

ref: N25B-165

src/control_backend/core/config.py

@@ -1,11 +1,16 @@
-from pydantic import HttpUrl
+from pydantic import BaseModel
 from pydantic_settings import BaseSettings, SettingsConfigDict
 
+class ZMQSettings(BaseModel):
+    internal_comm_address: str = "tcp://localhost:5560"
+
 class Settings(BaseSettings):
     app_title: str = "PepperPlus"
     
     ui_url: str = "http://localhost:5173"
+
+    zmq_settings: ZMQSettings = ZMQSettings()
     
     model_config = SettingsConfigDict(env_file=".env")
     
-settings = Settings()
+settings = Settings()

src/control_backend/core/zmq_context.py

@@ -0,0 +1,3 @@
+from zmq.asyncio import Context
+
+context = Context()