fix(x): drop chunked upload chunk size from 5MB to 1MB

Production was returning HTTP 413 with empty body on the first APPEND
segment of every video upload to X v2 — surfacing to users as 'An
unknown X error occurred.'

Empty-body 413 is the classic signature of an edge/CDN rejection: the
X gateway is denying the request before X's application code sees it.
The X v2 reference docs say 'max chunk size: 5MB', but every canonical
reference uses 1MB:
- X's official Python quickstart: `chunk_size = 1024 * 1024`
- X's official JavaScript quickstart: `const chunkSize = 1024 * 1024`
- twitter-api-v2 (the most-used Node SDK, used by Postiz et al.):
  `chunkSize: number = 1024 * 1024`

5MB plus multipart-form overhead apparently exceeds an undocumented
edge limit. 1MB is the empirically safe size everyone converges on.

Changes:
- XPublisher chunked APPEND now uses 1MB chunks. An 8MB video uploads
  as 8 segments instead of 2; more roundtrips but actually succeeds.
- Set explicit Content-Type on each chunk attach (matches the simple
  upload path in the same file).
- XPublishException::fromApiResponse maps HTTP 413 to
  ErrorCategory::MediaFormat with the message 'Media chunk rejected
  by X (payload too large).' so we don't surface 413 as 'unknown' if
  it ever recurs.

Test: unit test covering the 413→MediaFormat mapping. Full suite green
(1503 passed, 2 skipped).

app/Exceptions/Social/XPublishException.php

@@ -47,6 +47,15 @@ public static function fromApiResponse(mixed $response): static
             );
         }
 
+        if ($statusCode === 413) {
+            return new static(
+                userMessage: 'Media chunk rejected by X (payload too large).',
+                category: ErrorCategory::MediaFormat,
+                platformErrorCode: (string) $statusCode,
+                rawResponse: $rawResponse,
+            );
+        }
+
         if (in_array($statusCode, [500, 502, 503, 504], true)) {
             return new static(
                 userMessage: 'X server error. Please try again later.',

app/Services/Social/XPublisher.php

@@ -205,8 +205,11 @@ private function chunkedUpload(string $tempFile, int $totalBytes, string $mimeTy
             throw new \Exception('No media_id returned from INIT');
         }
 
-        // APPEND - Read from temp file in 5MB chunks (memory-safe)
-        $chunkSize = 5 * 1024 * 1024;
+        // APPEND - Read from temp file in 1MB chunks. Matches the
+        // twitter-api-v2 SDK default and X's own quickstart examples;
+        // larger chunks (we previously used 5MB) trigger 413 at the X
+        // edge with an empty body, surfacing as "An unknown X error".
+        $chunkSize = 1024 * 1024;
         $handle = fopen($tempFile, 'r');
         $index = 0;
 
@@ -220,7 +223,7 @@ private function chunkedUpload(string $tempFile, int $totalBytes, string $mimeTy
 
                 $appendResponse = $this->socialHttp()->withToken($this->accessToken)
                     ->timeout(300)
-                    ->attach('media', $chunk, 'chunk'.$index)
+                    ->attach('media', $chunk, 'chunk'.$index, ['Content-Type' => $mimeType])
                     ->post("{$this->baseUrl}/media/upload/{$mediaId}/append", [
                         'segment_index' => $index,
                     ]);

tests/Unit/Exceptions/Social/XPublishExceptionTest.php

@@ -77,6 +77,18 @@
         ->and($exception->userMessage)->toBe('X server error. Please try again later.');
 });
 
+test('HTTP 413 with empty body maps to MediaFormat category', function () {
+    $response = Http::response('', 413);
+
+    $fakeResponse = Http::fake(['*' => $response])->post('https://api.x.com/test');
+
+    $exception = XPublishException::fromApiResponse($fakeResponse);
+
+    expect($exception->category)->toBe(ErrorCategory::MediaFormat)
+        ->and($exception->userMessage)->toBe('Media chunk rejected by X (payload too large).')
+        ->and($exception->platformErrorCode)->toBe('413');
+});
+
 test('unknown type maps to Unknown category with detail as message', function () {
     $response = Http::response([
         'type' => 'https://api.x.com/2/problems/some-unknown-problem',