fix: align image dimensions to FLUX.1 divisible-by-8 requirement

FLUX.1 schnell requires width and height to be divisible by 8.
The default OG image size 1200×630 has 630 % 8 = 6 remainder,
causing HTTP 400 on every image generation call.

Changes:
- Pipeline default height 630 → 632 (nearest 8-aligned value)
- Provider now rounds any dimension to nearest multiple of 8 as
  a defensive guard before sending to the API

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

includes/core/class-image-pipeline.php

@@ -27,8 +27,10 @@ class PRAutoBlogger_Image_Pipeline {
 	/**
 	 * Default image dimensions (landscape).
 	 */
+	// FLUX.1 requires both dimensions to be divisible by 8.
+	// 1200×632 is the closest 8-aligned pair to the standard OG image size (1200×630).
 	private const DEFAULT_WIDTH  = 1200;
-	private const DEFAULT_HEIGHT = 630;
+	private const DEFAULT_HEIGHT = 632;
 
 	/**
 	 * The image provider (FLUX.1 via Cloudflare or another).