Soak's redis scenario refuses to run without --force

It stops the Redis the whole stack journals to, so every flow fails briefly,
not just the soak fixtures. `all` includes it and needs the flag too.

Makefile

@@ -204,7 +204,7 @@ test-frontend:  ## Run frontend tests (Playwright e2e) against the local stack
 
 # Load and chaos against a *running* stack, never part of `make test`: it
 # restarts this stack's containers. `SOAK_ARGS="--dry-run"` only looks.
-soak:  ## Run the soak/chaos harness (SOAK_ARGS="--minutes 30 --scenario redis")
+soak:  ## Run the soak/chaos harness (SOAK_ARGS="--minutes 30 --scenario redis --force")
 	cd backend && uv run python scripts/soak.py $(SOAK_ARGS)
 
 bench-startup:  ## Time submitting a run (BENCH_ARGS="--kedro ../some/kedro/project")

backend/scripts/soak.py

@@ -4,11 +4,14 @@ Standalone on purpose, and never part of the test run. It drives the running
 stack through the *durable* path — the webhook, which journals every trigger to
 the work queue — and then takes pieces of the stack away to see what survives.
 
-    make soak
-    make soak SOAK_ARGS="--minutes 30 --rps 25"
-    make soak SOAK_ARGS="--scenario redis"
+    make soak SOAK_ARGS="--force"
+    make soak SOAK_ARGS="--minutes 30 --rps 25 --scenario load"
+    make soak SOAK_ARGS="--scenario redis --force"
     make soak SOAK_ARGS="--dry-run"
 
+The redis scenario, which `all` includes, needs `--force`: the Redis it stops
+is the one the whole stack journals to, not a copy of its own.
+
 Every docker verb goes through :func:`docker`, which refuses any container that
 is not part of this project's compose stack. The machine this runs on hosts
 unrelated services; nothing here may ever reach them.
@@ -936,7 +939,18 @@ def parse_args(argv: list[str] | None) -> argparse.Namespace:
         action="store_true",
         help="Report what the harness can see and change nothing.",
     )
+    parser.add_argument(
+        "--force",
+        action="store_true",
+        help="Allow the redis scenario, which stops the Redis the whole stack shares.",
+    )
     args = parser.parse_args(argv)
+    if args.scenario in ("all", "redis") and not (args.force or args.dry_run):
+        parser.error(
+            f"the redis scenario stops {REDIS}, which the whole stack shares, so "
+            "every flow briefly fails to journal, not just the soak fixtures. "
+            "Pass --force on a stack nobody relies on, or pick another --scenario."
+        )
     args.rps = max(1, min(MAX_RPS, args.rps))
     args.seconds = max(10.0, args.minutes * 60)
     return args