feat(admin): search users by email + erase a bound email

Add an exact (strict) email filter to the /users list (UserFilter.EmailExact →
a kind='email' identity match) with a search input, and an 'Erase email' action on
the user card that deletes the bound email identity and its pending confirmations,
freeing the address. It refuses to remove the account's only identity
(ErrLastIdentity), which would leave it unreachable. Integration tests for both.

backend/internal/account/link.go

@@ -2,6 +2,7 @@ package account
 
 import (
 	"context"
+	"database/sql"
 	"errors"
 	"fmt"
 	"time"
@@ -16,6 +17,52 @@ import (
 // belongs to another account; the caller turns it into a merge.
 var ErrIdentityTaken = errors.New("account: identity already linked to another account")
 
+// ErrLastIdentity is returned when removing an identity would leave the account with
+// none, making it unreachable after logout. The admin email-erase refuses it.
+var ErrLastIdentity = errors.New("account: cannot remove the last identity")
+
+// RemoveEmailIdentity deletes the account's email identity and any pending confirmations
+// for it, freeing the address for reuse. It refuses when the email is the account's only
+// identity (ErrLastIdentity) — that would leave the account unreachable — and returns
+// ErrNotFound when the account has no email identity. It backs the admin console's
+// "erase email" action.
+func (s *Store) RemoveEmailIdentity(ctx context.Context, accountID uuid.UUID) error {
+	ids, err := s.Identities(ctx, accountID)
+	if err != nil {
+		return err
+	}
+	hasEmail, others := false, 0
+	for _, id := range ids {
+		if id.Kind == KindEmail {
+			hasEmail = true
+		} else {
+			others++
+		}
+	}
+	if !hasEmail {
+		return ErrNotFound
+	}
+	if others == 0 {
+		return ErrLastIdentity
+	}
+	return withTx(ctx, s.db, func(tx *sql.Tx) error {
+		delID := table.Identities.DELETE().WHERE(
+			table.Identities.AccountID.EQ(postgres.UUID(accountID)).
+				AND(table.Identities.Kind.EQ(postgres.String(KindEmail))),
+		)
+		if _, err := delID.ExecContext(ctx, tx); err != nil {
+			return fmt.Errorf("account: delete email identity %s: %w", accountID, err)
+		}
+		delConf := table.EmailConfirmations.DELETE().WHERE(
+			table.EmailConfirmations.AccountID.EQ(postgres.UUID(accountID)),
+		)
+		if _, err := delConf.ExecContext(ctx, tx); err != nil {
+			return fmt.Errorf("account: delete email confirmations %s: %w", accountID, err)
+		}
+		return nil
+	})
+}
+
 // RequestLinkCode issues and mails a confirm-code for email to accountID,
 // replacing any prior pending code. Unlike RequestCode it never refuses up front
 // (taken or already-confirmed): possession of the address is the authorization for