fix: pass editorial brief to LLM prompt + improve missing API key error

- Add 'brief' field to GenerateContentRequest schema
- Pass brief from router to generate_post_text service
- Inject brief as mandatory instructions in LLM prompt with highest priority
- Return structured error when LLM provider/API key not configured
- Show dedicated warning banner with link to Settings when API key missing

Fixes: content ignoring editorial brief, unhelpful API key error messages

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

frontend/src/components/ContentPage.jsx

@@ -85,7 +85,13 @@ export default function ContentPage() {
       setGenerated(data)
       setEditText(data.text_content || '')
     } catch (err) {
-      setError(err.message || 'Errore nella generazione')
+      if (err.data?.missing_settings) {
+        setError('__MISSING_SETTINGS__')
+      } else if (err.data?.upgrade_required) {
+        setError(err.data.message || 'Limite piano raggiunto. Passa a Pro.')
+      } else {
+        setError(err.message || 'Errore nella generazione')
+      }
     } finally {
       setLoading(false)
     }
@@ -140,11 +146,23 @@ export default function ContentPage() {
         </div>
       )}
 
-      {error && (
+      {error && (error === '__MISSING_SETTINGS__' ? (
+        <div style={{ padding: '1.25rem 1.5rem', backgroundColor: '#FFFBEB', border: '1px solid #FDE68A', borderLeft: '4px solid #F59E0B', marginBottom: '1rem' }}>
+          <div style={{ fontWeight: 600, fontSize: '0.9rem', color: '#92400E', marginBottom: '0.5rem' }}>
+            ⚙ Provider AI non configurato
+          </div>
+          <p style={{ fontSize: '0.85rem', color: '#78350F', margin: '0 0 0.75rem', lineHeight: 1.6 }}>
+            Per generare contenuti devi prima configurare un provider AI (Claude, OpenAI, Gemini...) e inserire la tua API key.
+          </p>
+          <Link to="/settings" style={{ ...btnPrimary, backgroundColor: '#F59E0B', textDecoration: 'none', fontSize: '0.82rem', padding: '0.5rem 1rem' }}>
+            Vai alle Impostazioni →
+          </Link>
+        </div>
+      ) : (
         <div style={{ padding: '0.75rem 1rem', backgroundColor: 'var(--error-light)', border: '1px solid #FED7D7', color: 'var(--error)', fontSize: '0.875rem', marginBottom: '1rem' }}>
           {error}
         </div>
-      )}
+      ))}
 
       <div style={{ display: 'grid', gridTemplateColumns: 'repeat(auto-fit, minmax(300px, 1fr))', gap: '1.25rem' }}>
         {/* Generation form */}