Require card_id for dashboard dashcard writes

[cdeil]

Summary

• require every dashboard dashcard write to include card_id
• continue to allow card_id: null for virtual cards
• document the requirement and cover it with a schema regression test

This makes incomplete whole-array dashboard updates fail locally before they can create a cardless duplicate. Restoring or preserving dashcards that reference archived questions still requires a Metabase core fix.

Agent authorship

This PR and its implementation were fully written by an AI coding agent.

Testing

• bun run check (118 test files, 1316 tests; typecheck, lint, formatting, and skill lint passed)
• disposable Metabase v0.63.15 verification: omitted card_id exits 1 before any HTTP request

Closes #49