Add EasyVerein invoicing and user syncing to the admin area

[nspo]

• Create draft or finalized invoices per user via a background job with a
  live progress page; mirror each as a pybarsys invoice + virtual payment,
  with rollback on failure and dependant purchase notifications.
• Sync pybarsys users to EasyVerein contacts via name matching with
  high/low-confidence suggestions; fetch and attach a contact directly from
  the user create/edit forms.
• Add a SiteSettings model (DB-stored API token with auto-refresh, bank
  account, finalize toggle), seeded once from PYBARSYS_INITIAL_* env vars.
• When EasyVerein is active: require self-paying buyers to be linked,
  enforce unique contact links, show link status, and redirect manual
  invoice/payment creation to the EasyVerein flow.
• Bump to Python 3.11 for python-easyverein.

---

Paginate user/by_account_balance correctly

Fixes #19

[nspo]

fyi @jallmenroeder if you wanna have a look

@Merlijnv We're adding an integration to easyverein.com but this is backwards-compatible with pybarsys-only-invoicing. The EasyVerein integration makes life much easier for the accountants, no need to create payments in Pybarsys any longer and re-invoice.

[nspo]

This is kind of how invoice creation looks now (slightly updated in most recent commit):

c.f. updated screenshot below

There's a live progress bar now, no more waiting forever for the browser to load.

PDF invoice sample:

[nspo]

Feature to automatically connect pybarsys users with EasyVerein contacts:

Better "invoice creation in progress" screenshot: