feat: add shared resource limits

[loonghao]

Summary

• Add shared resource/license limit definitions and snapshots through /v1/limits.
• Let tasks declare required named limits while keeping older payloads compatible.
• Gate scheduler leases by active limit usage and derive release from completion, failure, cancellation, or lease expiry.

Closes #8

Validation

• cargo fmt --all -- --check
• cargo test --workspace
• cargo clippy --workspace --all-targets -- -D warnings