feat: avatar upload via presigned PUT URL flow

- Go scraper: add PresignAvatarUploadURL/PresignAvatarURL/DeleteAvatar to
  Store interface, implement on HybridStore+MinioClient, register
  GET /api/presign/avatar-upload/{userId} and /api/presign/avatar/{userId}
- SvelteKit: replace direct AWS S3 SDK in minio.ts with presign calls to
  the Go scraper; rewrite avatar +server.ts (POST=presign, PATCH=record key)
- iOS: rewrite uploadAvatar() as 3-step presigned PUT flow; refactor
  chip components into shared ChipButton in CommonViews.swift

scraper/internal/storage/minio.go

@@ -362,6 +362,22 @@ func (m *MinioClient) PutAvatar(ctx context.Context, userID, ext string, data []
 	return nil
 }
 
+// PresignAvatarUploadURL returns a presigned PUT URL for uploading an avatar image
+// directly to MinIO from the client. Signed with the public endpoint so iOS/browser
+// can PUT bytes straight to MinIO without routing through the server.
+// ext should be "jpg", "png", or "webp". Expires in 15 minutes.
+func (m *MinioClient) PresignAvatarUploadURL(ctx context.Context, userID, ext string) (string, string, error) {
+	if m.cfg.BucketAvatars == "" {
+		return "", "", fmt.Errorf("minio: avatars bucket not configured")
+	}
+	key := avatarKey(userID, ext)
+	u, err := m.pub.PresignedPutObject(ctx, m.cfg.BucketAvatars, key, 15*time.Minute)
+	if err != nil {
+		return "", "", fmt.Errorf("minio: presign avatar upload %s: %w", key, err)
+	}
+	return u.String(), key, nil
+}
+
 // PresignAvatarURL returns a presigned GET URL for a user avatar.
 // Returns ("", false, nil) when no avatar exists for the given userID.
 func (m *MinioClient) PresignAvatarURL(ctx context.Context, userID string) (string, bool, error) {