Fix(dashboard): reject invalid reports_id when adding a widget

[Rom1-B]

Checklist before requesting a review

• I have performed a self-review of my code.
• I have added tests (when available) that prove my fix is effective or that my feature works.
• I have updated the CHANGELOG with a short functional description of the fix or new feature.
• This change requires a documentation update.

Description

• It fixes !44193

Submitting the "add report" form without selecting a report posted -1 (the empty dropdown sentinel value) into the reports_id column, which is defined UNSIGNED NOT NULL and rejects negative values. The form now validates the posted value before inserting. The migration purges any existing invalid rows and enforces the unsigned constraint on older installs.

Screenshots (if appropriate):