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/accountmerge/merge.go

@@ -27,6 +27,11 @@ import (
 // without taking a dependency on the game package.
 const statusActive = "active"
 
+// retainReasonMerge is the retained_identities.reason for a credential dropped by a merge
+// collision (both accounts held the same kind). It mirrors the account package's retain
+// reasons, kept local to avoid importing that package's unexported constants.
+const retainReasonMerge = "merge"
+
 // Friendship statuses, highest precedence first, mirroring internal/social.
 const (
 	friendAccepted = "accepted"
@@ -75,6 +80,9 @@ func (m *Merger) Merge(ctx context.Context, primary, secondary uuid.UUID) error
 		if err := mergeAccountFields(ctx, tx, primary, secondary, now); err != nil {
 			return err
 		}
+		if err := dedupeIdentities(ctx, tx, primary, secondary); err != nil {
+			return err
+		}
 		if err := reassignColumn(ctx, tx, table.Identities, table.Identities.AccountID, primary, secondary); err != nil {
 			return fmt.Errorf("accountmerge: identities: %w", err)
 		}
@@ -300,6 +308,77 @@ func reassignColumn(ctx context.Context, tx *sql.Tx, tbl postgres.Table, col pos
 	return err
 }
 
+// dedupeIdentities resolves a same-kind identity collision before the blanket identity
+// reassign: when both accounts already hold an identity of the same kind (e.g. each has a
+// confirmed email — reachable when two email-bearing accounts merge), the primary keeps
+// its own and the secondary's is journaled to retained_identities (reason=merge) and
+// removed. Without this the blanket reassign would leave the survivor with two identities
+// of one kind (there is no per-account-kind unique on identities), which the profile and
+// the retention dossier both treat as singular. Non-colliding identities are untouched and
+// move with the blanket reassign.
+func dedupeIdentities(ctx context.Context, tx *sql.Tx, primary, secondary uuid.UUID) error {
+	var prows []model.Identities
+	if err := postgres.SELECT(table.Identities.Kind).
+		FROM(table.Identities).
+		WHERE(table.Identities.AccountID.EQ(postgres.UUID(primary))).
+		QueryContext(ctx, tx, &prows); err != nil && !errors.Is(err, qrm.ErrNoRows) {
+		return fmt.Errorf("accountmerge: primary identity kinds: %w", err)
+	}
+	occupied := make(map[string]struct{}, len(prows))
+	for _, r := range prows {
+		occupied[r.Kind] = struct{}{}
+	}
+	if len(occupied) == 0 {
+		return nil
+	}
+	var srows []model.Identities
+	if err := postgres.SELECT(
+		table.Identities.Kind, table.Identities.ExternalID,
+		table.Identities.Confirmed, table.Identities.CreatedAt,
+	).FROM(table.Identities).
+		WHERE(table.Identities.AccountID.EQ(postgres.UUID(secondary))).
+		QueryContext(ctx, tx, &srows); err != nil && !errors.Is(err, qrm.ErrNoRows) {
+		return fmt.Errorf("accountmerge: secondary identities: %w", err)
+	}
+	for _, s := range srows {
+		if _, dup := occupied[s.Kind]; !dup {
+			continue
+		}
+		if err := retainMergedIdentity(ctx, tx, secondary, s); err != nil {
+			return err
+		}
+		del := table.Identities.DELETE().WHERE(
+			table.Identities.AccountID.EQ(postgres.UUID(secondary)).
+				AND(table.Identities.Kind.EQ(postgres.String(s.Kind))).
+				AND(table.Identities.ExternalID.EQ(postgres.String(s.ExternalID))),
+		)
+		if _, err := del.ExecContext(ctx, tx); err != nil {
+			return fmt.Errorf("accountmerge: drop colliding %s identity: %w", s.Kind, err)
+		}
+	}
+	return nil
+}
+
+// retainMergedIdentity appends a retained_identities row for a secondary identity dropped
+// by a merge collision (reason=merge), preserving it in the legal dossier. It mirrors
+// account.retainIdentityTx, which is unexported; detached_at falls to the column default.
+func retainMergedIdentity(ctx context.Context, tx *sql.Tx, accountID uuid.UUID, id model.Identities) error {
+	rid, err := uuid.NewV7()
+	if err != nil {
+		return fmt.Errorf("accountmerge: new retained id: %w", err)
+	}
+	ins := table.RetainedIdentities.INSERT(
+		table.RetainedIdentities.RetainedID, table.RetainedIdentities.AccountID,
+		table.RetainedIdentities.Kind, table.RetainedIdentities.ExternalID,
+		table.RetainedIdentities.Confirmed, table.RetainedIdentities.LinkedAt,
+		table.RetainedIdentities.Reason,
+	).VALUES(rid, accountID, id.Kind, id.ExternalID, id.Confirmed, id.CreatedAt, retainReasonMerge)
+	if _, err := ins.ExecContext(ctx, tx); err != nil {
+		return fmt.Errorf("accountmerge: retain merged %s identity: %w", id.Kind, err)
+	}
+	return nil
+}
+
 // friendRank ranks a friendship status for dedupe precedence (higher wins).
 func friendRank(status string) int {
 	switch status {