Separate comic captions from generated images

Captions were being baked into the image by the FLUX model, causing
text to get cut off at the edges. Now the LLM generates scene and
caption separately — the scene drives image gen (with explicit
"no text in the image" instruction), and the caption is inserted
as a styled <figure>/<figcaption> HTML block at the top of the post.

Changes:
- rewrite_via_llm() returns array{scene, caption} instead of string
- build_article_prompt() and build_source_prompt() return {prompt, caption}
- Style suffix v4: removes "caption text" instruction, adds "no text" directive
- Image pipeline destructures prompt/caption, stores caption in attachment meta
- prepend_caption_to_post() inserts figure+figcaption HTML into post content
- Deduplicated generate_image_a/b into shared generate_single_image()
- One-time v4 migration auto-updates existing style suffix on deploy

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

includes/class-prautoblogger.php

@@ -159,6 +159,17 @@ class PRAutoBlogger {
 			update_option( 'prautoblogger_migrated_style_suffix_v3', '1' );
 		}
 
+		// One-time migration (v4): remove caption-in-image instruction from style suffix.
+		// Captions are now inserted as HTML below the image, not baked into the image.
+		if ( ! get_option( 'prautoblogger_migrated_style_suffix_v4' ) ) {
+			$current = get_option( 'prautoblogger_image_style_suffix', '' );
+			// Detect v3 style suffix (contains "caption text" instruction).
+			if ( false !== strpos( $current, 'caption text in a clean sans-serif font below the panel' ) ) {
+				update_option( 'prautoblogger_image_style_suffix', PRAUTOBLOGGER_DEFAULT_IMAGE_STYLE_SUFFIX );
+			}
+			update_option( 'prautoblogger_migrated_style_suffix_v4', '1' );
+		}
+
 		// One-time migration: re-wrap existing encrypted values with "enc:" prefix.
 		if ( ! get_option( 'prautoblogger_migrated_enc_prefix' ) ) {
 			$enc_options = [ 'prautoblogger_openrouter_api_key', 'prautoblogger_ga4_credentials_json' ];