Fix deleting courses and codes (coursecodes FK constraint)

[VoiToi]

Fixes #1237

Deleting a course in Classes → Courses or a code in Classes → Codes failed on the coursecodes foreign key constraint (ON DELETE RESTRICT), because both dialogs used a plain SqlTableModel which only runs DELETE FROM courses/codes WHERE id=….

Changes

• Courses (EditCoursesWidget): deleting a course deletes its coursecodes rows in the same transaction. A course still assigned to a class (classdefs.courseId) or a run (runs.courseId) is not deleted, the user gets a readable message (class names + stage, or run count) instead of an SQL error.
• Codes (EditCodesWidget): a code used in a course is not deleted, the message lists the courses using it (first 10 + "and N more").
• The checks run in dropRows() before beginRemoveRows(), so a refused delete does not leave the view showing the row as removed.
• Confirmation: new TableView::setRemoveRowsQuestion() to replace the generic "Do you really want to remove row?" question. Courses ask "Do you really want to delete N course(s) including their codes? This cannot be undone.", codes ask a similar question. Default button stays No.
• Czech translations for the new strings.

Testing

• Built with Qt 6.11.2 / MinGW 13.1 (Release), no warnings in the changed files.
• Root cause verified on a real relay event DB (PostgreSQL, 1077 courses / 124 codes / 21700 coursecodes): no other table references courses/codes except coursecodes, classdefs.courseId, runs.courseId.
• The new delete paths have not been clicked through in the running app yet.

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