feat: enhance relation permissions and accessibility checks

[enjeck]

This PR has the following improvements:

• Relation columns can now only link to tables or views you actually have access to, so you can't link to (and expose data from) something you can't see.
• The target picker now hides tables/views you can't read and the table the column lives in, so you only choose valid targets.
• If a linked table or view stops being accessible (e.g. it gets unshared), the relation is automatically disabled and its data hidden, instead of leaking stale information.
• A clear warning now appears on a relation whose target is no longer accessible, telling you to ask for access or pick a different target (right now, the column ID is shown with the warning, but we could just make it empty instead)

• Newly added or edited relation columns show their linked data immediately, without needing a page refresh.

🏁 Checklist

• ⛑️ Tests (unit and/or integration) are included or not needed
• 🔙 Backport requests are created or not needed: /backport to stableX.X
• 📅 Milestone is set
• 🌸 PR title is meaningful (if it should be in the changelog: is it meaningful to users?)

🤖 AI (if applicable)

• The content of this PR was partly or fully generated using AI