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/adapter.py

@@ -34,11 +34,25 @@ class C2TaskStatus:
     status: str
     completed: bool
     completed_at: datetime | None = field(default=None)
+    # command_name is populated by get_task() so import doesn't need a second round-trip.
+    command: str | None = field(default=None)
+
+
+@dataclass
+class C2HistoricalTask:
+    """A task entry from callback history (carries command + params, unlike C2TaskStatus)."""
+
+    display_id: int
+    command: str
+    params: str | None
+    status: str
+    completed: bool
+    timestamp: str | None  # ISO-8601 or None
 
 
 @dataclass
 class C2TaskPage:
-    items: list[dict]  # raw task dicts from Mythic
+    items: list[C2HistoricalTask]
     total: int
     page: int
     page_size: int