fix(uploads): switch from S3 POST policy to presigned PUT

Backblaze B2's S3-compatible endpoint does not implement the S3 POST
Object operation. It returns HTTP 501 to every POST regardless of URL
style — both path-style (https://s3.<region>.backblazeb2.com/<bucket>/)
and virtual-hosted-style (https://<bucket>.s3.<region>.backblazeb2.com/).

Yesterday's BucketLookupDNS fix produced virtual-hosted URLs, which is
correct for AWS but doesn't help here — B2 rejects POST on either form.
Verified with `curl -X POST https://...backblazeb2.com/honeyDueProd/`
returning 501 directly, with no signature involved.

Replace minio-go's PresignedPostPolicy with PresignHeader + http.MethodPut.
The signed URL now points at a single PUT endpoint, with Content-Type
and Content-Length signed via headers — B2/S3/MinIO all accept it. Drop
the min/max content-length range (we sign exactly one length now);
post-upload size verification still happens in VerifyAndClaim via HEAD.

Response shape:
  - URL  (was: signed POST endpoint)  → now: signed PUT URL
  - Fields → renamed to Headers; client sends them as request headers,
    not multipart form parts
  - Method (new): always "PUT", emitted explicitly so clients don't
    have to hardcode

Companion KMP/iOS commits switch the client paths from multipart POST
to single PUT. Existing builds in the field will need to be rebuilt.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/services/upload_service.go

@@ -164,18 +164,12 @@ func (s *UploadService) Presign(
 	ext := extensionForContentType(contentType)
 	key := fmt.Sprintf("uploads/%s/%d/%s%s", subdir, userID, uuid.New().String(), ext)
 
-	// Sign the POST policy. The slack window lets the client encode once;
-	// the server still rejects anything materially different from claimed.
-	minB := contentLength - UploadPresignSlackBytes
-	if minB < 0 {
-		minB = 0
-	}
-	maxB := contentLength + UploadPresignSlackBytes
-	if maxB > UploadMaxBytes {
-		maxB = UploadMaxBytes
-	}
-
-	post, err := s.s3.PresignedPost(ctx, key, contentType, minB, maxB, UploadPresignTTL)
+	// Sign a single-use PUT URL that binds Content-Type and Content-Length.
+	// B2 does not implement S3's POST Object form upload (returns 501), so
+	// we use PUT — which works uniformly against AWS S3, B2, and MinIO.
+	// Size mismatch is caught later in VerifyAndClaim via HEAD; we don't
+	// need a min/max range here because the signature pins exactly one size.
+	put, err := s.s3.PresignedPut(ctx, key, contentType, contentLength, UploadPresignTTL)
 	if err != nil {
 		return nil, apperrors.Internal(fmt.Errorf("presign upload: %w", err))
 	}
@@ -195,8 +189,9 @@ func (s *UploadService) Presign(
 
 	return &responses.PresignUploadResponse{
 		ID:        row.ID,
-		URL:       post.URL,
-		Fields:    post.Fields,
+		URL:       put.URL,
+		Headers:   put.Headers,
+		Method:    "PUT",
 		Key:       key,
 		ExpiresAt: expiresAt.Format(time.RFC3339),
 	}, nil