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>

prautoblogger.php

@@ -60,11 +60,12 @@ define( 'PRAUTOBLOGGER_DEFAULT_IMAGE_PROVIDER', 'openrouter' );
 define( 'PRAUTOBLOGGER_DEFAULT_IMAGE_MODEL', 'google/gemini-2.5-flash-image' );
 
 // Default style suffix appended to every image-gen prompt. Updated
-// 2026-04-18: single-panel newspaper comic style. Users may override
-// via Settings → Images → Style Suffix.
+// 2026-04-18 v4: single-panel newspaper comic style. Caption is now
+// inserted as HTML text below the image, NOT rendered inside it.
+// Users may override via Settings → Images → Style Suffix.
 define(
 	'PRAUTOBLOGGER_DEFAULT_IMAGE_STYLE_SUFFIX',
-	'Style: a single-panel newspaper comic in classic black ink on white paper, simple clean linework like The Far Side or B.C., slightly yellowed newsprint texture, hand-drawn crosshatching for shading, bold panel border, caption text in a clean sans-serif font below the panel, warm and humorous tone, visible in a newspaper comics section.'
+	'Style: a single-panel newspaper comic in classic black ink on white paper, simple clean linework like The Far Side or B.C., slightly yellowed newsprint texture, hand-drawn crosshatching for shading, bold panel border, warm and humorous tone, visible in a newspaper comics section. IMPORTANT: Do not render any text, captions, speech bubbles, or words inside the image.'
 );
 
 /*