feat(backend): c2 callback history + task import (sprint 8 M4)

Command source decision: extended C2TaskStatus with command: str | None
(default None). Added command_name to _GET_TASK_QUERY so get_task() returns
command in a single round-trip — no separate history fetch needed on import.
4-line change, zero cascading test impact.

adapter.py:
- C2TaskStatus: add command: str | None = None field
- C2HistoricalTask: new dataclass (display_id, command, params, status,
  completed, timestamp) for history rows
- C2TaskPage.items: typed as list[C2HistoricalTask] (was list[dict])

mythic.py:
- _GET_TASK_QUERY: add command_name field
- _LIST_CALLBACK_TASKS_QUERY: new query (order_by id desc, limit/offset)
- _COUNT_CALLBACK_TASKS_QUERY: new aggregate query for total
- get_task(): surfaces command_name as status.command
- list_callback_tasks(): two _post() calls (tasks + count), allow_redirects=False

fake.py:
- _FAKE_HISTORY: frozen deterministic history (cb1=12, cb2=0, cb3=5 tasks)
- list_callback_tasks(): serves from _FAKE_HISTORY, pagination applied
- get_task(): returns command from _tasks dict

api/c2.py:
- GET /api/engagements/<eid>/c2/callbacks/<cid>/history: page+page_size
  defaults 1/25, cap 100, reject <1, 502 on adapter error
- POST /api/simulations/<sid>/c2/import: idempotent per (sim,mythic_id) pair,
  source=import, completed tasks get output+mapping_applied, incomplete tasks
  stored for poll-on-read pickup, auto-transition pending→in_progress

60 new tests (456 total); pytest/ruff/mypy all green

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/services/c2/mythic.py

@@ -21,6 +21,7 @@ from backend.app.services.c2.adapter import (
     C2Callback,
     C2Error,
     C2Health,
+    C2HistoricalTask,
     C2TaskPage,
     C2TaskStatus,
     decode_response_text,
@@ -61,6 +62,7 @@ _GET_TASK_QUERY = """
 query GetTask($display_id: Int!) {
   task(where: {display_id: {_eq: $display_id}}) {
     display_id
+    command_name
     status
     completed
     timestamp
@@ -68,6 +70,34 @@ query GetTask($display_id: Int!) {
 }
 """
 
+_LIST_CALLBACK_TASKS_QUERY = """
+query ListCallbackTasks($callback_display_id: Int!, $limit: Int!, $offset: Int!) {
+  task(
+    where: {callback: {display_id: {_eq: $callback_display_id}}}
+    order_by: {id: desc}
+    limit: $limit
+    offset: $offset
+  ) {
+    display_id
+    command_name
+    params
+    status
+    completed
+    timestamp
+  }
+}
+"""
+
+_COUNT_CALLBACK_TASKS_QUERY = """
+query CountCallbackTasks($callback_display_id: Int!) {
+  task_aggregate(where: {callback: {display_id: {_eq: $callback_display_id}}}) {
+    aggregate {
+      count
+    }
+  }
+}
+"""
+
 _GET_TASK_OUTPUT_QUERY = """
 query GetTaskOutput($display_id: Int!) {
   response(
@@ -197,6 +227,7 @@ class MythicAdapter(C2Adapter):
             status=row["status"],
             completed=bool(row.get("completed", False)),
             completed_at=completed_at,
+            command=row.get("command_name") or None,
         )
 
     def get_task_output(self, task_display_id: int) -> str:
@@ -222,4 +253,41 @@ class MythicAdapter(C2Adapter):
         page: int = 1,
         page_size: int = 25,
     ) -> C2TaskPage:
-        raise NotImplementedError("M4")
+        """Return a paginated, most-recent-first history of tasks for a callback."""
+        offset = (page - 1) * page_size
+        try:
+            data = self._post({
+                "query": _LIST_CALLBACK_TASKS_QUERY,
+                "variables": {
+                    "callback_display_id": callback_display_id,
+                    "limit": page_size,
+                    "offset": offset,
+                },
+            })
+            count_data = self._post({
+                "query": _COUNT_CALLBACK_TASKS_QUERY,
+                "variables": {"callback_display_id": callback_display_id},
+            })
+        except requests.RequestException as exc:
+            raise C2Error(str(exc)) from exc
+
+        rows = data.get("data", {}).get("task", [])
+        total: int = (
+            count_data.get("data", {})
+            .get("task_aggregate", {})
+            .get("aggregate", {})
+            .get("count", 0)
+        )
+
+        items = [
+            C2HistoricalTask(
+                display_id=r["display_id"],
+                command=r.get("command_name") or "",
+                params=r.get("params") or None,
+                status=r.get("status") or "",
+                completed=bool(r.get("completed", False)),
+                timestamp=r.get("timestamp") or None,
+            )
+            for r in rows
+        ]
+        return C2TaskPage(items=items, total=total, page=page, page_size=page_size)