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/mapping.py

@@ -0,0 +1,38 @@
+"""C2 task → Simulation output mapping.
+
+apply_task_to_simulation() writes task output into the simulation's
+execution_result field and marks the task as mapping_applied=True so that
+the operation is idempotent (safe to call multiple times for the same task).
+
+Caller is responsible for committing the session.
+"""
+from __future__ import annotations
+
+from datetime import UTC, datetime
+
+from backend.app.models.c2_task import C2Task
+from backend.app.models.simulation import Simulation
+
+
+def apply_task_to_simulation(task: C2Task, simulation: Simulation) -> None:
+    """Write task output into simulation.execution_result (append, newline-separated).
+
+    No-op if task.mapping_applied is already True or task.output is empty.
+    Marks task.mapping_applied = True on completion.
+    """
+    if task.mapping_applied:
+        return
+
+    output = (task.output or "").strip()
+    if not output:
+        task.mapping_applied = True
+        return
+
+    existing = (simulation.execution_result or "").rstrip("\n")
+    if existing:
+        simulation.execution_result = existing + "\n" + output
+    else:
+        simulation.execution_result = output
+
+    simulation.updated_at = datetime.now(UTC)
+    task.mapping_applied = True