feat: add e2e test infrastructure with k3d and Camunda 8

[lenaschoenburg]

Summary

Adds end-to-end test infrastructure for testing c8-backup against a real Kubernetes cluster with Camunda 8.

Changes

• lib+bin restructure: Split main.rs into lib.rs + thin main.rs so integration tests can import crate modules directly
• E2E tests (ES mode): tests/e2e_es.rs — tests list and create commands against a Camunda 8 cluster with Elasticsearch secondary storage
• E2E tests (RDBMS mode): tests/e2e_rdbms.rs — tests list and create commands against a Camunda 8 cluster with PostgreSQL + filesystem backup store
• k3d setup scripts: scripts/e2e-setup.sh and scripts/e2e-teardown.sh for local cluster management
• CI workflow: .github/workflows/e2e.yml with two parallel jobs (ES and RDBMS), each deploying a k3d cluster with the appropriate Camunda 8 configuration
• Test Helm values: Minimal Camunda 8 deployment values for both modes

Local usage

./scripts/e2e-setup.sh           # Create k3d cluster + deploy Camunda
cargo test --test e2e_es -- --nocapture --ignored   # Run ES tests
cargo test --test e2e_rdbms -- --nocapture --ignored # Run RDBMS tests
./scripts/e2e-teardown.sh        # Cleanup

Design decisions

• Tests use #[ignore] and skip gracefully when no cluster is available, so cargo test still works without a cluster
• 22 existing unit tests continue to pass unchanged
• 4 new e2e tests are ignored by default (require live cluster)

Test plan

• cargo build passes
• cargo test — 22 passed, 4 ignored
• cargo fmt --check clean
• E2E tests pass on k3d cluster (CI will validate)