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/tests/test_migration_0007_c2.py

@@ -0,0 +1,124 @@
+"""Migration round-trip test for 0007_c2_task_mapping_applied.
+
+Verifies that upgrade() adds the mapping_applied column and downgrade() removes it.
+Uses the resolved-path pattern per lessons.md Sprint 4.
+"""
+from __future__ import annotations
+
+import importlib.util
+from pathlib import Path
+
+from alembic.operations import Operations
+from alembic.runtime.migration import MigrationContext
+from sqlalchemy import create_engine, inspect, text
+
+
+def _load_migration(name: str):
+    versions_dir = Path(__file__).resolve().parent.parent / "migrations" / "versions"
+    path = versions_dir / name
+    spec = importlib.util.spec_from_file_location(name.removesuffix(".py"), path)
+    assert spec and spec.loader
+    mod = importlib.util.module_from_spec(spec)
+    spec.loader.exec_module(mod)  # type: ignore[union-attr]
+    return mod
+
+
+def _fresh_engine_with_c2_task():
+    """In-memory SQLite with c2_task already created (as left by 0006 upgrade)."""
+    engine = create_engine("sqlite:///:memory:")
+    with engine.begin() as conn:
+        conn.execute(text("""
+            CREATE TABLE c2_task (
+                id INTEGER PRIMARY KEY,
+                simulation_id INTEGER NOT NULL,
+                mythic_task_display_id INTEGER NOT NULL,
+                callback_display_id INTEGER NOT NULL,
+                command TEXT NOT NULL,
+                params TEXT,
+                status TEXT NOT NULL,
+                completed BOOLEAN NOT NULL DEFAULT 0,
+                output TEXT,
+                source TEXT NOT NULL,
+                created_at DATETIME NOT NULL,
+                completed_at DATETIME
+            )
+        """))
+    return engine
+
+
+def _run_upgrade(engine, migration_mod):
+    with engine.begin() as conn:
+        ctx = MigrationContext.configure(conn)
+        ops = Operations(ctx)
+        ops._install_proxy()  # type: ignore[attr-defined]
+        try:
+            migration_mod.upgrade()
+        finally:
+            ops._remove_proxy()  # type: ignore[attr-defined]
+
+
+def _run_downgrade(engine, migration_mod):
+    with engine.begin() as conn:
+        ctx = MigrationContext.configure(conn)
+        ops = Operations(ctx)
+        ops._install_proxy()  # type: ignore[attr-defined]
+        try:
+            migration_mod.downgrade()
+        finally:
+            ops._remove_proxy()  # type: ignore[attr-defined]
+
+
+class TestMigration0007Upgrade:
+    def test_mapping_applied_column_added(self):
+        engine = _fresh_engine_with_c2_task()
+        mod = _load_migration("0007_c2_task_mapping_applied.py")
+        _run_upgrade(engine, mod)
+
+        insp = inspect(engine)
+        cols = {c["name"] for c in insp.get_columns("c2_task")}
+        assert "mapping_applied" in cols
+
+    def test_mapping_applied_defaults_to_false(self):
+        engine = _fresh_engine_with_c2_task()
+        mod = _load_migration("0007_c2_task_mapping_applied.py")
+
+        # Insert a row before upgrading (no mapping_applied column yet).
+        with engine.begin() as conn:
+            conn.execute(text(
+                "INSERT INTO c2_task "
+                "(simulation_id, mythic_task_display_id, callback_display_id, "
+                "command, status, completed, source, created_at) "
+                "VALUES (1, 1000, 1, 'whoami', 'submitted', 0, 'mimic', '2026-01-01')"
+            ))
+
+        _run_upgrade(engine, mod)
+
+        with engine.begin() as conn:
+            row = conn.execute(
+                text("SELECT mapping_applied FROM c2_task WHERE id = 1")
+            ).fetchone()
+        assert row is not None
+        # SQLite stores booleans as 0/1.
+        assert row[0] == 0 or row[0] is False
+
+
+class TestMigration0007Downgrade:
+    def test_downgrade_removes_mapping_applied(self):
+        engine = _fresh_engine_with_c2_task()
+        mod = _load_migration("0007_c2_task_mapping_applied.py")
+        _run_upgrade(engine, mod)
+        _run_downgrade(engine, mod)
+
+        insp = inspect(engine)
+        cols = {c["name"] for c in insp.get_columns("c2_task")}
+        assert "mapping_applied" not in cols
+
+    def test_downgrade_does_not_drop_other_columns(self):
+        engine = _fresh_engine_with_c2_task()
+        mod = _load_migration("0007_c2_task_mapping_applied.py")
+        _run_upgrade(engine, mod)
+        _run_downgrade(engine, mod)
+
+        insp = inspect(engine)
+        cols = {c["name"] for c in insp.get_columns("c2_task")}
+        assert {"id", "simulation_id", "command", "status", "completed"} <= cols