[FIX] document_page_reference: avoid duplicate reference on copy and enforce uniqueness

[miquelrosell99]

Before this fix:

• Duplicating a document.page copied the reference value as-is, so the new page ended up with the same reference as the original.
• The module only had a Python uniqueness check; there was no database-level unique constraint.

This fix:

• Sets copy=False on the reference field so copies get a fresh reference generated from the copied page name.
• Adds a SQL unique constraint on reference.
• Includes a pre-migration script that clears duplicate references (keeping the oldest record) so the constraint can be applied on databases that already have duplicates.
• Adds a regression test.

[OCA-git-bot]

Hi @etobella,
some modules you are maintaining are being modified, check this out!