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/migrations/versions/0007_c2_task_mapping_applied.py

@@ -0,0 +1,30 @@
+"""add mapping_applied column to c2_task
+
+Revision ID: 0007
+Revises: 0006
+Create Date: 2026-06-10 00:00:00.000000
+"""
+import sqlalchemy as sa
+from alembic import op
+
+revision = "0007"
+down_revision = "0006"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    with op.batch_alter_table("c2_task") as batch_op:
+        batch_op.add_column(
+            sa.Column(
+                "mapping_applied",
+                sa.Boolean(),
+                nullable=False,
+                server_default=sa.false(),
+            )
+        )
+
+
+def downgrade() -> None:
+    with op.batch_alter_table("c2_task") as batch_op:
+        batch_op.drop_column("mapping_applied")