feat: image generation integrated into content creation flow

Backend:
- When content_types includes "image", generate image via configured
  image provider (DALL-E, Replicate) after text generation
- Image prompt built from character niche + topic + visual style
- Graceful fallback: if image generation fails, post is text-only
- Post content_type set to "text+image" when image is attached

Frontend:
- ContentPage preview shows generated image above text when present
- Image displayed with full width and border

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/ContentPage.jsx

@@ -442,6 +442,10 @@ export default function ContentPage() {
                 </div>
               ) : (
                 <div style={{ padding: '1rem', backgroundColor: 'var(--cream)', marginBottom: '1rem' }}>
+                  {generated.image_url && (
+                    <img src={generated.image_url} alt="Immagine generata"
+                      style={{ width: '100%', marginBottom: '0.75rem', border: '1px solid var(--border)' }} />
+                  )}
                   <p style={{ fontSize: '0.875rem', whiteSpace: 'pre-wrap', lineHeight: 1.7, color: 'var(--ink)', margin: 0 }}>
                     {generated.text_content}
                   </p>