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/database.py

@@ -1,10 +1,13 @@
-from sqlalchemy import text
-from sqlalchemy.ext.asyncio import create_async_engine, AsyncSession, async_sessionmaker
-from sqlalchemy.orm import DeclarativeBase
-from app.config import settings
+from collections.abc import AsyncGenerator
 import os
 import re
 
+from sqlalchemy import text
+from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker, create_async_engine
+from sqlalchemy.orm import DeclarativeBase
+
+from app.config import settings
+
 os.makedirs(settings.DATA_DIR, exist_ok=True)
 
 engine = create_async_engine(
@@ -24,12 +27,9 @@ class Base(DeclarativeBase):
     pass
 
 
-async def get_db() -> AsyncSession:
+async def get_db() -> AsyncGenerator[AsyncSession, None]:
     async with async_session() as session:
-        try:
-            yield session
-        finally:
-            await session.close()
+        yield session
 
 
 async def init_db():
@@ -37,6 +37,7 @@ async def init_db():
         await conn.run_sync(Base.metadata.create_all)
         await ensure_log_columns(conn)
         await ensure_message_columns(conn)
+        await ensure_conversation_columns(conn)
         await ensure_document_columns(conn)
         await ensure_user_columns(conn)
         await ensure_forum_columns(conn)
@@ -79,6 +80,20 @@ async def ensure_message_columns(conn):
             await conn.execute(text(ddl))
 
 
+async def ensure_conversation_columns(conn):
+    rows = await _get_table_info(conn, 'conversations')
+    if not rows:
+        return
+
+    columns = {row[1] for row in rows}
+    required_columns = {
+        'agent_state': "ALTER TABLE conversations ADD COLUMN agent_state JSON",
+    }
+    for column, ddl in required_columns.items():
+        if column not in columns:
+            await conn.execute(text(ddl))
+
+
 async def ensure_document_columns(conn):
     result = await conn.execute(text("PRAGMA table_info(documents)"))
     rows = result.fetchall()