…in localstack

`make up/test` broke after the dev-stack unify (#930) removed the external
`outpost` network and `make network`. The test compose still pinned its
default network to `external: outpost`, so the project failed to start.

The test suite reaches every service via mapped localhost ports (.env.test),
never by container DNS, so the shared network was never load-bearing. Drop
the override and let the `outpost-test` project auto-create
`outpost-test_default` — which also matches what redis-cluster-compose expects.

Also pin localstack to `:3`; `:latest` now resolves to a 2026.x build that
gates startup on a Pro license token. The 3.x community line runs
s3/sns/sts/sqs/kinesis free.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

The e2e basic config forced a timestamped signature template
(t={{.Timestamp.Unix}},v0=...) left over from before #808. That refactor
removed timestamps from the default templates and updated the mock server
to verify the bare default (v0= over {{.Body}}), but never updated this
override — so signing and verification diverged.

The mismatch was masked for months because compose reuses the cached
`outpost-mock` image; it only surfaced on a fresh mock rebuild. Drop the
override so e2e exercises the real product default and matches the mock.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>