Ticket: README: fix stale reference to deleted bff/internal/webankverify/client.go

## Story Statement

As a reader of this README, I want the "Relationship to webank-kyc-manager" section to
stop describing a caller that no longer exists, so the archival record doesn't point
future readers at deleted code.

## Real Intent

`README.md`'s "Relationship to webank-kyc-manager (deprecated)" section currently reads
(verified 2026-08-08, `README.md` line 26):

> the BFF uses a separate `webankverify` client (`bff/internal/webankverify/client.go`)
> pointed at `WEBANK_VERIFY_BASE_URL`.

`bff/internal/webankverify/client.go` no longer exists — the entire Go BFF tree
(`bff/`) was deleted outright in `webank-mobile`#318, not just that one client. The
caller into `webank-verify` today is `webank-services/app/bff` (Rust), not a Go client
at that path.

## Source of truth (links)

- `webank-verify/README.md` line 26 (the stale reference, verified 2026-08-08)
- `webank-mobile`#318 (the Go BFF deletion — `bff/` including
`bff/internal/webankverify/` no longer exists in any form)
- `webank-services/app/bff` (the current Rust caller)

## Acceptance criteria, risks, and test plan

- [ ] README's "Relationship to webank-kyc-manager" section updated to name the current
Rust `app/bff` caller instead of the deleted Go client path, or to drop the
file-path detail entirely if it's not load-bearing for the section's point.
- [ ] No other stale `bff/internal/*` references remain elsewhere in the README (grep
the whole file, not just this section, before closing).
- Risk: low — documentation-only change, but worth doing before archival since this
README is the artifact most likely to be read after the repo is archived and no
longer maintained.

## Human accountable owner

@stephane-segning

## AI Usage Declaration

Understanding code, Proposing implementation

## Human verification

- [x] I understood the intent and checked the source of truth.
- [x] I reviewed AI-generated material as untrusted, where used.
- [ ] I am the accountable owner and accept responsibility for this ticket.