Fix duplicate image, remove hardcoded model references

Three fixes in one:

1. Duplicate image: prepend_caption_to_post() was embedding the full
   image + caption as a <figure> block, but the theme already displays
   the featured image. Now only the caption text is prepended — no
   second copy of the image.

2. Hardcoded model names: removed all FLUX.1/FLUX.2 references from
   model-agnostic code (pipeline, prompt builder, sideloader, OpenRouter
   provider, interface, admin labels). The Cloudflare provider still
   references FLUX since it specifically wraps that model family.

3. Test alignment: OpenRouterImagePricingTest was asserting against
   removed models (flux-2-pro, seedream). Updated to match current
   COST_PER_IMAGE entries (Gemini Flash, GPT-5 Image).

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

includes/providers/interface-image-provider.php

@@ -13,7 +13,7 @@ declare(strict_types=1);
  *               content generation run, after editorial review, before publish.
  * Dependencies: None at interface level — implementations wrap a specific HTTP API.
  *
- * @see class-cloudflare-image-provider.php — Default implementation (FLUX.1 via Workers AI).
+ * @see class-cloudflare-image-provider.php — Cloudflare Workers AI implementation.
  * @see interface-llm-provider.php          — Parallel pattern for LLM providers.
  * @see ARCHITECTURE.md                     — External API Integrations table.
  */