refactor: centralize avatar URL generation and backfill existing users

internal/database/fixes/20260528_backfill_avatar_url.go

@@ -0,0 +1,46 @@
+package fixes
+
+import (
+	"context"
+	"database/sql"
+	"fmt"
+	"mal/internal/users"
+)
+
+func init() {
+	Register(Fix{
+		ID: "20260528_backfill_avatar_url",
+		Apply: func(ctx context.Context, sqlDB *sql.DB) error {
+			rows, err := sqlDB.QueryContext(ctx, `SELECT id, username FROM user WHERE avatar_url = ''`)
+			if err != nil {
+				return err
+			}
+			defer func() { _ = rows.Close() }()
+
+			type userRow struct {
+				id       string
+				username string
+			}
+			toUpdate := make([]userRow, 0, 64)
+			for rows.Next() {
+				var r userRow
+				if err := rows.Scan(&r.id, &r.username); err != nil {
+					return err
+				}
+				toUpdate = append(toUpdate, r)
+			}
+			if err := rows.Err(); err != nil {
+				return err
+			}
+
+			for _, u := range toUpdate {
+				avatarURL := users.DefaultAvatarURL(u.username)
+				if _, err := sqlDB.ExecContext(ctx, `UPDATE user SET avatar_url = ? WHERE id = ?`, avatarURL, u.id); err != nil {
+					return fmt.Errorf("update avatar_url for user %s: %w", u.id, err)
+				}
+			}
+
+			return nil
+		},
+	})
+}