Add admin ability to change user passwords

[OlivierJM]

Feature Parity: Admin Password Management

Legacy app allows admins to set/change passwords for any user; sparked-next only has user-initiated forgot-password flow.

Legacy capabilities

• Admin can set a new password for any user account
• Accessible from the user management admin page
• No email required — direct password override
• Useful for onboarding users or recovering locked accounts

Acceptance criteria

• PUT /api/admin/users/:id/password — admin sets new password for a user
• Password is bcrypt-hashed before storage
• Admin UI: password change form on user edit page (admin dashboard)
• Confirm password field to prevent typos
• Action restricted to admin role only
• Optional: notify user via email that their password was reset

Identified via feature-parity audit against legacy Sparked (Meteor) app.