### Description
This PR introduces native, secure support for using Cloud SQL as the
PostgreSQL store backend for `ate-api-server`.
To ensure the highest level of security and ease of use in GCP
environments, this integration leverages the Cloud SQL Auth Proxy
sidecar with automatic IAM database authentication. This means transport
security (TLS 1.3 tunnel) is handled automatically, and database
sessions are authenticated using Workload Identity via short-lived OAuth
tokens, completely eliminating the need for database passwords.
### Key Changes
* **Cloud SQL Auth Proxy Sidecar:** Added
`manifests/ate-install/cloudsql-proxy-patch.yaml` to patch the sidecar
into the `ate-api-server` deployment when a Cloud SQL instance is
configured.
* **Automated Provisioning:** Extended `tools/setup-gcp` with a new
`cloudsql` command. This handles the idempotent creation of the Cloud
SQL instance, Google Service Accounts (GSA), IAM bindings, and Workload
Identity bindings.
* **Installation Script Updates:** Updated `hack/install-ate.sh` to
parse new environment variables (e.g.,
`ATE_API_POSTGRES_CLOUDSQL_INSTANCE`, `ATE_API_POSTGRES_CLOUDSQL_GSA`)
and correctly synthesize the passwordless DSN and ConfigMaps for the
proxy.
* **Security & Documentation:**
* Added extensive documentation in `tools/setup-gcp/cloud-sql.md`
covering provisioning, schema privileges, deployment, and database
scaling.
* Updated `docs/threat-model.md` to reflect the new Cloud SQL egress
flows and Auth Proxy tunnel mechanics.
* **Dependencies:** Vendored required Google API clients (`sqladmin/v1`,
`servicenetworking/v1`, `iam/v1`) for the GCP setup tool.
- [X] Tests pass
- [X] Appropriate changes to documentation are included in the PR
