feat(backend): c2 poll-on-read + output mapping (sprint 8 M3)

- adapter.py: add completed_at field to C2TaskStatus dataclass
- mythic.py: implement get_task() (GraphQL task query) and
  get_task_output() (response query + decode_response_text concat)
- fake.py: deterministic state progression via per-instance call counter;
  get_task_output raises C2Error until completed
- mapping.py: apply_task_to_simulation() idempotent output mapper
  (mapping_applied anchor prevents double-writes)
- migration 0007: add mapping_applied BOOLEAN NOT NULL DEFAULT false to c2_task
- c2_task model: mapping_applied column added
- api/c2.py: GET /api/simulations/<id>/c2/tasks poll-on-read endpoint;
  refreshes incomplete tasks from C2, fetches output on completion,
  applies mapping, skips re-polling for completed tasks; best-effort
  (C2Error on individual task skipped, returns 200 with stale status)
- 51 new tests (396 total); pytest/ruff/mypy all green

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

backend/app/services/c2/fake.py

@@ -8,6 +8,7 @@ from __future__ import annotations
 from backend.app.services.c2.adapter import (
     C2Adapter,
     C2Callback,
+    C2Error,
     C2Health,
     C2TaskPage,
     C2TaskStatus,
@@ -46,11 +47,17 @@ class FakeAdapter(C2Adapter):
     """In-memory adapter with deterministic behaviour.
 
     Each instance starts with an empty task store and display_ids from 1000.
+
+    get_task() state progression per task (keyed by display_id):
+      - First call after create_task → submitted, completed=False
+      - Second and subsequent calls → completed=True, status="completed"
     """
 
     def __init__(self) -> None:
         self._tasks: dict[int, dict] = {}
         self._next_task_id = 1000
+        # Tracks how many times get_task has been called per display_id.
+        self._get_task_calls: dict[int, int] = {}
 
     def test_connection(self) -> C2Health:
         return C2Health(ok=True)
@@ -78,20 +85,44 @@ class FakeAdapter(C2Adapter):
         return tid
 
     def get_task(self, task_display_id: int) -> C2TaskStatus:
+        """Deterministic state progression: first call → submitted, second+ → completed.
+
+        Tracks call count regardless of whether the task was created by this instance,
+        so the endpoint poll-on-read flow works across separate adapter instantiations.
+        """
+        call_count = self._get_task_calls.get(task_display_id, 0) + 1
+        self._get_task_calls[task_display_id] = call_count
+
         task = self._tasks.get(task_display_id)
-        if task is None:
-            return C2TaskStatus(display_id=task_display_id, status="unknown", completed=False)
+
+        if call_count >= 2:
+            completed = True
+            status = "completed"
+            if task is not None:
+                task["status"] = "completed"
+                task["completed"] = True
+        else:
+            completed = False
+            status = task["status"] if task is not None else "submitted"
+
         return C2TaskStatus(
             display_id=task_display_id,
-            status=task["status"],
-            completed=task["completed"],
+            status=status,
+            completed=completed,
         )
 
     def get_task_output(self, task_display_id: int) -> str:
+        """Returns deterministic output once task is completed; raises C2Error before that."""
+        # Check call count — completed if get_task was called at least twice.
+        if self._get_task_calls.get(task_display_id, 0) < 2:
+            # Also allow tasks in _tasks that were explicitly set to completed.
+            task = self._tasks.get(task_display_id)
+            if task is None or not task.get("completed", False):
+                raise C2Error("task not completed")
+
         task = self._tasks.get(task_display_id)
-        if task is None:
-            return ""
-        return task.get("output") or ""
+        command = task["command"] if task is not None else "unknown"
+        return f"output for task {task_display_id}: {command}\n"
 
     def list_callback_tasks(
         self,