fix: Fix YAML syntax errors and validator prompt formatting

- Fix YAML files with unquoted strings containing quotes
- Export singleton instances in ai/__init__.py
- Fix validator scoring prompt to use replace() instead of format()
  to avoid conflicts with JSON curly braces

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/services/ai/validator.py

@@ -261,11 +261,8 @@ class ContentValidator:
         if not scoring_prompt:
             scoring_prompt = self._default_scoring_prompt()
 
-        # Renderizar prompt
-        prompt = scoring_prompt.format(
-            content=content,
-            platform=platform
-        )
+        # Renderizar prompt usando replace para evitar problemas con JSON
+        prompt = scoring_prompt.replace("{content}", content).replace("{platform}", platform)
 
         # Llamar a DeepSeek
         response = self.client.chat.completions.create(
@@ -340,7 +337,7 @@ CRITERIOS (suma = 100):
 - CTA (0-10): ¿CTA claro si aplica?
 
 RESPONDE EN JSON:
-{{"total": N, "breakdown": {{"hook_strength": N, "clarity": N, "actionability": N, "originality": N, "brand_voice": N, "cta_effectiveness": N}}, "feedback": "sugerencia"}}"""
+{"total": N, "breakdown": {"hook_strength": N, "clarity": N, "actionability": N, "originality": N, "brand_voice": N, "cta_effectiveness": N}, "feedback": "sugerencia"}"""
 
     def _extract_score_from_text(self, text: str) -> Dict:
         """Extraer score de texto si falla JSON parsing."""