Partner-specific mentee application links

[Odzen]

Replaces old issue #926.

Problem

Partners need a link they can share with mentees so the application form automatically selects the correct partner affiliation. This reduces incorrect organization selection during application.

Required behavior

• Generate a shareable application link for each partner.
• Opening the link preselects the partner in the mentee application form.
• The user should not accidentally submit with the wrong partner after using a partner-specific link.
• Existing generic application links must keep working.

Acceptance criteria

• Partner or admin can copy a partner-specific mentee application link.
• Link includes a stable partner identifier or slug.
• Mentee application form preselects the partner from the link.
• If the partner id is invalid or inactive, the form shows a persistent visible error or falls back safely.
• Submitted application stores the intended partner id.