fix(account): dedupe colliding identities on merge, journaling to the dossier

An account merge blanket-reassigned all of the secondary's identities to the primary,
so merging two accounts that each had a confirmed email (or telegram/vk) left the
survivor with two identities of one kind — which the profile and the retention dossier
both treat as singular (the profile showed an arbitrary one; a later change-email
journaled only one). The merge now keeps the primary's identity and journals the
secondary's colliding one to retained_identities (reason=merge) before dropping it, so
the survivor has one identity per kind and the absorbed credential still lands in the
legal dossier.

- migration 00008: widen retained_identities.reason CHECK to admit 'merge'
  (expand-contract — Up only widens the set, so an image rollback stays DB-safe).
- accountmerge: dedupeIdentities + retainMergedIdentity before the identity reassign.
- inttest TestAccountMergeDedupesEmail; docs ARCHITECTURE §4 + retention.go reason note.

backend/internal/inttest/merge_test.go

@@ -251,6 +251,44 @@ func TestAccountMergeFinishedSharedGameKept(t *testing.T) {
 	}
 }
 
+// TestAccountMergeDedupesEmail keeps the primary's email when both accounts have one and
+// journals the secondary's to the dossier (reason=merge), so the survivor never ends up
+// with two email identities.
+func TestAccountMergeDedupesEmail(t *testing.T) {
+	ctx := context.Background()
+	store := account.NewStore(testDB)
+	merger := accountmerge.NewMerger(testDB)
+
+	primary := provisionAccount(t)
+	secondary := provisionAccount(t)
+	primaryEmail := "keep-" + uuid.NewString() + "@example.com"
+	secondaryEmail := "absorb-" + uuid.NewString() + "@example.com"
+	bindEmailIdentity(t, primary, primaryEmail)
+	bindEmailIdentity(t, secondary, secondaryEmail)
+
+	if err := merger.Merge(ctx, primary, secondary); err != nil {
+		t.Fatalf("merge: %v", err)
+	}
+
+	// The primary keeps its own email; the secondary's is gone from the live identities.
+	if owner, ok, _ := store.AccountIDByIdentity(ctx, account.KindEmail, primaryEmail); !ok || owner != primary {
+		t.Errorf("primary email owner = %s ok=%v, want primary %s", owner, ok, primary)
+	}
+	if _, ok, _ := store.AccountIDByIdentity(ctx, account.KindEmail, secondaryEmail); ok {
+		t.Error("the secondary's email must be removed (no duplicate email on the survivor)")
+	}
+	// The absorbed email stays in the legal dossier, tagged reason=merge.
+	var reason string
+	if err := testDB.QueryRowContext(ctx,
+		`SELECT reason FROM backend.retained_identities WHERE account_id=$1 AND kind='email' AND external_id=$2`,
+		secondary, secondaryEmail).Scan(&reason); err != nil {
+		t.Fatalf("retained email row: %v", err)
+	}
+	if reason != "merge" {
+		t.Errorf("retained reason = %q, want merge", reason)
+	}
+}
+
 // TestAccountLinkFreeEmail binds a free email and promotes a guest to durable.
 func TestAccountLinkFreeEmail(t *testing.T) {
 	ctx := context.Background()