test: verify api graceful shutdown ordering

[Goodnessoj]

Closes #161

Summary

• Extracted the API graceful shutdown sequence into a small testable helper.
• Documented the intended order: stop HTTP listener first, cancel/wait workers, drain/close bus, close DB last.
• Added tests for shutdown ordering, DB remaining open until HTTP drain completes, repeated shutdown invocation, dependency close errors, and non-fatal worker wait timeout handling.

Tests

• go test ./cmd/api/... -v

Notes

• Existing HTTP shutdown timeout behavior remains delegated to internal/shutdown.Wait for workers and internal/api.Shutdown for Fiber.
• Current bus and DB close methods do not return errors, but the extracted helper preserves error reporting for close hooks so future close failures are not masked.