test: complete manager change end-to-end workflow with confirmation

[mrofreP]

TestManagerChange_EndToEndWorkflow/complete_manager_change_workflow
verified steps 1-3 (request creation, email delivery via mailpit,
status endpoint) and then stopped at step 4 with:

t.Log("Skipping confirmation step - panic in ConfirmManagerChange
needs separate fix")

The referenced panic is unrelated to this workflow — server-generated
tokens are always long enough to slice safely — so the workflow can
be completed without waiting on that fix.

Extend the test with steps 4 and 5:

• Step 4: read the confirmation token directly from pending_mgrchange
  via dbPool.QueryRow (parsing it out of the mailpit HTML would be
  fragile), then drive GET /channels/:id/manager-confirm?token=... and
  assert 200 + status=confirmed + change_type=temporary in the
  response body.
• Step 5: verify the DB row's confirmed column is now '1'.

Net effect: the manager-change happy path is now covered end-to-end
against a real Postgres + real mailpit, closing the loop from
submission through email delivery to confirmation.

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

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