feat: system messages for task lifecycle (create, assign, status change)

Tracker now auto-generates system messages visible to all members/agents.
No more side effects in router — agents see system messages and act via tools.

src/tracker/api/tasks.py

@@ -80,6 +80,33 @@ class AssignRequest(BaseModel):
 
 # --- Helpers ---
 
+async def _system_message(db: AsyncSession, task: Task, content: str, project_slug: str = ""):
+    """Create a system message for a task and broadcast it via WS."""
+    from tracker.ws.manager import manager
+    msg = Message(
+        task_id=task.id,
+        author_type="system",
+        author_slug="system",
+        content=content,
+        mentions=[],
+    )
+    db.add(msg)
+    await db.flush()  # get msg.id
+
+    prefix = project_slug[:2].upper() if project_slug else "XX"
+    key = f"{prefix}-{task.number}"
+
+    await manager.broadcast_task_event(str(task.project_id), "message.new", {
+        "id": str(msg.id),
+        "task_id": str(task.id),
+        "task_key": key,
+        "author_type": "system",
+        "author_slug": "system",
+        "content": content,
+        "created_at": msg.created_at.isoformat() if msg.created_at else "",
+    })
+
+
 def _task_out(t: Task, project_slug: str = "") -> dict:
     prefix = project_slug[:2].upper() if project_slug else "XX"
     return {
@@ -200,13 +227,28 @@ async def create_task(
         "created_at": task.created_at.isoformat() if task.created_at else "",
     }
     await manager.broadcast_task_event(str(project.id), "task.created", task_data)
-    
-    return _task_out(task_full, task_full.project.slug if task_full.project else "")
+
+    # System message
+    slug = project.slug
+    key = f"{slug[:2].upper()}-{task.number}"
+    sys_text = f"Задача {key} создана: {task.title}"
+    if task.assignee_slug:
+        sys_text += f". Назначена на @{task.assignee_slug}"
+    await _system_message(db, task_full, sys_text, slug)
+    await db.commit()
+
+    return _task_out(task_full, slug)
 
 
 @router.patch("/tasks/{task_id}", response_model=TaskOut)
 async def update_task(task_id: str, req: TaskUpdate, db: AsyncSession = Depends(get_db)):
     task = await _get_task(task_id, db)
+    slug = task.project.slug if task.project else ""
+    prefix = slug[:2].upper() if slug else "XX"
+    key = f"{prefix}-{task.number}"
+
+    old_status = task.status
+    old_assignee = task.assignee_slug
 
     if req.title is not None:
         task.title = req.title
@@ -227,6 +269,15 @@ async def update_task(task_id: str, req: TaskUpdate, db: AsyncSession = Depends(
     if req.position is not None:
         task.position = req.position
 
+    # System messages for important changes
+    if req.status is not None and req.status != old_status:
+        await _system_message(db, task, f"{key}: статус изменён {old_status} → {req.status}", slug)
+    if req.assignee_slug is not None and req.assignee_slug != old_assignee:
+        if req.assignee_slug:
+            await _system_message(db, task, f"{key}: назначена на @{req.assignee_slug}", slug)
+        else:
+            await _system_message(db, task, f"{key}: исполнитель снят (был @{old_assignee})", slug)
+
     await db.commit()
     await db.refresh(task)
     
@@ -240,7 +291,7 @@ async def update_task(task_id: str, req: TaskUpdate, db: AsyncSession = Depends(
     }
     await manager.broadcast_task_event(str(task.project_id), "task.updated", task_data)
     
-    return _task_out(task, task.project.slug if task.project else "")
+    return _task_out(task, slug)
 
 
 @router.delete("/tasks/{task_id}")
@@ -271,6 +322,13 @@ async def take_task(task_id: str, slug: str = Query(...), db: AsyncSession = Dep
     await db.commit()
     await db.refresh(task)
     
+    # System message
+    proj_slug = task.project.slug if task.project else ""
+    prefix = proj_slug[:2].upper() if proj_slug else "XX"
+    key = f"{prefix}-{task.number}"
+    await _system_message(db, task, f"{key}: @{slug} взял задачу в работу", proj_slug)
+    await db.commit()
+
     # Broadcast task.assigned event
     from tracker.ws.manager import manager
     task_data = {
@@ -281,7 +339,7 @@ async def take_task(task_id: str, slug: str = Query(...), db: AsyncSession = Dep
     }
     await manager.broadcast_task_event(str(task.project_id), "task.assigned", task_data)
     
-    return _task_out(task, task.project.slug if task.project else "")
+    return _task_out(task, proj_slug)
 
 
 @router.post("/tasks/{task_id}/reject")
@@ -323,6 +381,13 @@ async def assign_task(task_id: str, req: AssignRequest, db: AsyncSession = Depen
     await db.commit()
     await db.refresh(task)
     
+    # System message
+    proj_slug = task.project.slug if task.project else ""
+    prefix = proj_slug[:2].upper() if proj_slug else "XX"
+    key = f"{prefix}-{task.number}"
+    await _system_message(db, task, f"{key}: назначена на @{req.assignee_slug}", proj_slug)
+    await db.commit()
+
     # Broadcast task.assigned event  
     from tracker.ws.manager import manager
     task_data = {
@@ -333,7 +398,7 @@ async def assign_task(task_id: str, req: AssignRequest, db: AsyncSession = Depen
     }
     await manager.broadcast_task_event(str(task.project_id), "task.assigned", task_data)
     
-    return _task_out(task, task.project.slug if task.project else "")
+    return _task_out(task, proj_slug)
 
 
 @router.post("/tasks/{task_id}/watch")