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

backend/internal/session/cache.go

@@ -4,6 +4,8 @@ import (
 	"context"
 	"sync"
 	"sync/atomic"
+
+	"github.com/google/uuid"
 )
 
 // Cache is the in-memory write-through projection of the active rows in
@@ -93,3 +95,19 @@ func (c *Cache) Remove(tokenHash string) {
 	defer c.mu.Unlock()
 	delete(c.byHash, tokenHash)
 }
+
+// RemoveByAccount evicts every cached session belonging to accountID. The
+// account-merge flow uses it to drop a retired secondary account's sessions
+// (Stage 11); a linear scan is adequate at the cache's size.
+func (c *Cache) RemoveByAccount(accountID uuid.UUID) {
+	if c == nil {
+		return
+	}
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	for hash, s := range c.byHash {
+		if s.AccountID == accountID {
+			delete(c.byHash, hash)
+		}
+	}
+}