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

[paulocastellano]

Context

Every video post to X has been failing in production with the cryptic error 'An unknown X error occurred.' Logs show the pattern:

```
production.ERROR: X chunked upload APPEND error {"status":413,"body":"","segment":0}
production.ERROR: Social publish failed: An unknown X error occurred.
```

Status 413 with empty body on the very first chunk — the classic signature of an edge/CDN rejection (the gateway is denying the request before X's application code sees it).

Root cause

We were using 5MB chunks (max documented size per X v2 reference), 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 and the broader ecosystem): `chunkSize: number = 1024 * 1024` as the default

The docs say 5MB max, but with multipart-form overhead, the actual HTTP request body exceeds an undocumented edge limit. 1MB is the empirically safe size everyone converges on.

Changes

1. XPublisher.php:209 — drop chunk size from `5 * 1024 * 1024` to `1024 * 1024`. An 8MB video uploads as 8 APPEND segments instead of 2; more roundtrips but actually succeeds.

2. XPublisher.php:224 — set explicit `Content-Type` on each chunk attach (matches the simple-upload path in the same file at line 144-151).

3. `XPublishException::fromApiResponse` — map 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 for any reason.

Test plan

• `php artisan test --compact --parallel` — 1503 passed, 2 skipped, 0 failed (+1 over main, the new 413 mapping test)
• New unit test: `HTTP 413 with empty body maps to MediaFormat category`
• Manual production verification: post an 8MB video to X via the production app after deploy → confirm successful publish (no more 'unknown' errors)