feat: add agent visibility APIs and harden runtime verification

Add Day 4 visibility endpoints and response models, strengthen collaboration/task verification behavior, and patch conversation schema startup migration for agent_state compatibility. Extend backend regression coverage for runtime schemas, verifier behavior, visibility APIs, router auth, and legacy conversation list loading.

backend/app/agents/schemas/message.py

@@ -0,0 +1,29 @@
+from __future__ import annotations
+
+from datetime import datetime, timezone
+from typing import Any, Literal
+
+from pydantic import BaseModel, Field
+
+
+AgentMessageType = Literal[
+    "task_request",
+    "task_update",
+    "handoff",
+    "verification_request",
+    "verification_feedback",
+    "interrupt_notice",
+]
+
+
+class AgentMessage(BaseModel):
+    message_id: str
+    thread_id: str
+    from_agent_id: str
+    to_agent_id: str
+    task_id: str | None = None
+    reply_to_message_id: str | None = None
+    message_type: AgentMessageType = "task_update"
+    content_summary: str
+    created_at: datetime = Field(default_factory=lambda: datetime.now(timezone.utc))
+    payload: dict[str, Any] = Field(default_factory=dict)