MistDemo: --database flag + error demonstration commands (talk prep)

[leogdion]

Parent tracking issue for two paired MistDemo changes needed for the "CloudKit as Your Backend" talk. These should land together (or sequentially in the order below) because the new error-demo commands from #269 should honor the --database flag introduced in #259 from day one.

Sub-issues

• Commands should accept --database flag instead of hardcoding database #259 — Commands should accept --database flag instead of hardcoding database
• MistDemo: Add error demonstration commands for talk demo #269 — MistDemo: Add error demonstration commands for talk demo

Order

1. Commands should accept --database flag instead of hardcoding database #259 first — establishes --database on MistDemoConfig (or per-command), following the TestIntegrationConfig pattern.
2. MistDemo: Add error demonstration commands for talk demo #269 second — new demo-errors command(s) for 401/404/409 scenarios, built on top of the new flag so error demos can target public vs. private as needed (e.g. 401 differs between server-to-server auth and web auth).

Why bundle

• Avoids re-touching the new error commands once --database lands.
• Error scenarios are auth/database-sensitive, so the flag is directly useful for the demo.
• Both are blocking the talk run-through (originally May 4, 2026).