feat(account): VK ID web login to link a VK identity from a browser

A browser has no signed VK Mini App launch params, so linking VK on the web uses
VK ID's raw OAuth 2.1 flow (PKCE, no @vkid/sdk): the SPA redirects to VK's hosted
login and returns with an authorization code, which the gateway exchanges
server-side (confidential, under the VK "Web" app's protected key) for the trusted
vk user id — then the existing link/merge machinery attaches or merges it.

- fbs LinkVKRequest{code, device_id, code_verifier}; codec + TS bindings.
- backend link.Service ConfirmVK/MergeVK/attachVK (KindVK, mirror Telegram),
  handleLinkVK[Merge], routes /user/link/vk[/merge], backendclient LinkVK[Merge].
- gateway internal/vkid confidential code exchange (id.vk.com/oauth2/auth);
  transcode link.vk.confirm/merge (registered only when configured) + config
  GATEWAY_VK_ID_{APP_ID,CLIENT_SECRET,REDIRECT_URL} + main wiring.
- UI lib/vkid (PKCE authorize redirect + callback), Profile "Link VK" control,
  boot callback handling; a merge re-authorizes for a fresh code (VK codes are
  single-use). Web-only (a redirect strands a Mini App webview).
- Deploy: VITE_VK_APP_ID + VITE_VK_ID_REDIRECT_URL build args + gateway env,
  ci.yaml/prod-deploy TEST_/PROD_ vars, compose/Dockerfile/.env.example/README.
- Tests: vkid exchange unit (string/number user_id, id_token fallback, errors),
  transcode link.vk, backend ConfirmVK/MergeVK inttest, codec encodeLinkVK.
- Docs: ARCHITECTURE §4, FUNCTIONAL(+ru), gateway README.

backend/internal/link/service.go

@@ -133,6 +133,53 @@ func (s *Service) attachTelegram(ctx context.Context, callerID uuid.UUID, extern
 	return s.accounts.ClearGuest(ctx, callerID)
 }
 
+// ConfirmVK attaches a gateway-validated VK identity to the caller (Linked) or
+// reports that it belongs to another account (MergeRequired). The gateway has already
+// completed the VK ID code exchange, so externalID is the trusted vk user id.
+func (s *Service) ConfirmVK(ctx context.Context, callerID uuid.UUID, externalID string) (ConfirmResult, error) {
+	owner, ok, err := s.accounts.AccountIDByIdentity(ctx, account.KindVK, externalID)
+	if err != nil {
+		return ConfirmResult{}, err
+	}
+	if !ok {
+		if err := s.attachVK(ctx, callerID, externalID); err != nil {
+			return ConfirmResult{}, err
+		}
+		return ConfirmResult{Linked: true}, nil
+	}
+	if owner == callerID {
+		return ConfirmResult{Linked: true}, nil
+	}
+	return ConfirmResult{MergeRequired: true, SecondaryID: owner}, nil
+}
+
+// MergeVK merges the account owning a gateway-validated VK identity into the caller's
+// (subject to the guest-primary rule).
+func (s *Service) MergeVK(ctx context.Context, callerID uuid.UUID, externalID string) (MergeResult, error) {
+	owner, ok, err := s.accounts.AccountIDByIdentity(ctx, account.KindVK, externalID)
+	if err != nil {
+		return MergeResult{}, err
+	}
+	if !ok {
+		if err := s.attachVK(ctx, callerID, externalID); err != nil {
+			return MergeResult{}, err
+		}
+		return MergeResult{PrimaryID: callerID}, nil
+	}
+	if owner == callerID {
+		return MergeResult{PrimaryID: callerID}, nil
+	}
+	return s.merge(ctx, callerID, owner)
+}
+
+// attachVK links the identity to the caller and promotes a guest.
+func (s *Service) attachVK(ctx context.Context, callerID uuid.UUID, externalID string) error {
+	if err := s.accounts.AttachIdentity(ctx, callerID, account.KindVK, externalID, true); err != nil {
+		return err
+	}
+	return s.accounts.ClearGuest(ctx, callerID)
+}
+
 // merge decides the primary (the caller, unless it is a guest and the other is
 // durable), runs the data merge, retires the secondary's sessions and mints a new
 // session when the active account switches.