Stage 11: account linking & merge (email + Telegram Login Widget)

Link an email (confirm-code) or Telegram (web Login Widget) to the current
account; if the identity already has its own account, merge the two into the
one in use (the current account is primary, except a guest initiator whose
durable counterpart wins). The merge runs in one transaction
(internal/accountmerge): stats + hint wallet summed, paid_account ORed,
identities/games/chat/complaints transferred, friends/blocks de-duplicated,
the secondary kept as a merged_into tombstone so a shared finished game's
no-cascade FKs hold; a shared active game blocks the merge.

- migration 00009: accounts.paid_account, merged_into, merged_at (+ jetgen)
- internal/link orchestrator; session.RevokeAllForAccount on merge
- connector ValidateLoginWidget RPC + loginwidget HMAC validator
- edge ops link.email.request/confirm/merge, link.telegram.confirm/merge;
  supersedes the Stage 8 email.bind.* surface (request never reveals 'taken'
  before the code is verified, so a probe cannot enumerate addresses)
- UI Profile link section + irreversible-merge dialog; Telegram web sign-in
- focused regression tests (merge core, guest inversion, active-game refusal,
  finished-shared-game kept), gateway transcode + connector + UI codec/e2e
- docs: PLAN, ARCHITECTURE 3/4/9, FUNCTIONAL(+ru), module READMEs

gateway/internal/backendclient/api_social.go

@@ -228,20 +228,47 @@ func (c *Client) UpdateProfile(ctx context.Context, userID string, p ProfileResp
 	return out, err
 }
 
-// EmailBindRequest asks the backend to mail a confirm-code binding email.
-func (c *Client) EmailBindRequest(ctx context.Context, userID, email string) error {
-	return c.do(ctx, http.MethodPost, "/api/v1/user/email/request", userID, "",
+// LinkEmailRequest asks the backend to mail a confirm-code for a link or merge.
+func (c *Client) LinkEmailRequest(ctx context.Context, userID, email string) error {
+	return c.do(ctx, http.MethodPost, "/api/v1/user/link/email/request", userID, "",
 		map[string]string{"email": email}, nil)
 }
 
-// EmailBindConfirm verifies the code and binds the email, returning the profile.
-func (c *Client) EmailBindConfirm(ctx context.Context, userID, email, code string) (ProfileResp, error) {
-	var out ProfileResp
-	err := c.do(ctx, http.MethodPost, "/api/v1/user/email/confirm", userID, "",
+// LinkEmailConfirm verifies the code and binds a free email or reports a required
+// merge (Stage 11).
+func (c *Client) LinkEmailConfirm(ctx context.Context, userID, email, code string) (LinkResultResp, error) {
+	var out LinkResultResp
+	err := c.do(ctx, http.MethodPost, "/api/v1/user/link/email/confirm", userID, "",
 		map[string]string{"email": email, "code": code}, &out)
 	return out, err
 }
 
+// LinkEmailMerge re-verifies the code and performs the merge (Stage 11).
+func (c *Client) LinkEmailMerge(ctx context.Context, userID, email, code string) (LinkResultResp, error) {
+	var out LinkResultResp
+	err := c.do(ctx, http.MethodPost, "/api/v1/user/link/email/merge", userID, "",
+		map[string]string{"email": email, "code": code}, &out)
+	return out, err
+}
+
+// LinkTelegram attaches a gateway-validated Telegram identity to the caller or
+// reports a required merge (Stage 11).
+func (c *Client) LinkTelegram(ctx context.Context, userID, externalID string) (LinkResultResp, error) {
+	var out LinkResultResp
+	err := c.do(ctx, http.MethodPost, "/api/v1/user/link/telegram", userID, "",
+		map[string]string{"external_id": externalID}, &out)
+	return out, err
+}
+
+// LinkTelegramMerge merges the account owning a gateway-validated Telegram identity
+// into the caller's (Stage 11).
+func (c *Client) LinkTelegramMerge(ctx context.Context, userID, externalID string) (LinkResultResp, error) {
+	var out LinkResultResp
+	err := c.do(ctx, http.MethodPost, "/api/v1/user/link/telegram/merge", userID, "",
+		map[string]string{"external_id": externalID}, &out)
+	return out, err
+}
+
 // Stats returns the caller's lifetime statistics.
 func (c *Client) Stats(ctx context.Context, userID string) (StatsResp, error) {
 	var out StatsResp