Write integration test verifying SDK client against live backend

[Jayy4rl]

Context: The generated SDK client (Issue 382) must be validated end-to-end against the running backend, not just at the type level.

Requirements:

• Create sdk/packages/backend-client/src/__tests__/integration.test.ts.
• Start the backend with docker-compose.test.yml in beforeAll.
• Test client.GET("/api/v1/health") returns { status: "ok" }.
• Test client.GET("/api/v1/vaults") returns a valid vault list parseable by VaultSchema.
• Skip (not fail) when BACKEND_URL env var is not set.

Acceptance criteria:

• Integration tests pass against the real backend and test database.
• Tests are skipped gracefully when the backend URL is not configured.