feat: Add Content Generation Engine v2 with quality scoring

Major improvements to AI content generation:

## New Components (app/services/ai/)
- PromptLibrary: YAML-based prompt templates with inheritance
- ContextEngine: Anti-repetition and best performers tracking
- ContentGeneratorV2: Enhanced generation with dynamic parameters
- PlatformAdapter: Platform-specific content adaptation
- ContentValidator: AI-powered quality scoring (0-100)

## Prompt Library (app/prompts/)
- 3 personalities: default, educational, promotional
- 5 templates: tip_tech, product_post, service_post, thread, response
- 4 platform configs: x, threads, instagram, facebook
- Few-shot examples by category: ia, productividad, seguridad

## Database Changes
- New table: content_memory (tracks generated content)
- New columns in posts: quality_score, score_breakdown, generation_attempts

## New API Endpoints (/api/v2/generate/)
- POST /generate - Generation with quality check
- POST /generate/batch - Batch generation
- POST /quality/evaluate - Evaluate content quality
- GET /templates, /personalities, /platforms - List configs

## Celery Tasks
- update_engagement_scores (every 6h)
- cleanup_old_memory (monthly)
- refresh_best_posts_yaml (weekly)

## Tests
- Comprehensive tests for all AI engine components

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

app/prompts/personalities/educational.yaml

@@ -0,0 +1,37 @@
+name: educational
+description: Personalidad didáctica para hilos y contenido educativo extenso
+
+extends: default  # Hereda de default y sobrescribe
+
+voice:
+  tone: teacher_mentor
+  formality: medium
+  expertise_level: expert_teacher
+
+personality_traits:
+  - patient         # Explica sin prisa
+  - thorough        # Cubre el tema completo
+  - encouraging     # Motiva al aprendizaje
+  - structured      # Organiza la información
+
+communication_style:
+  sentence_length: medium
+  paragraph_style: structured
+  use_examples: always
+  use_analogies: when_helpful
+
+teaching_techniques:
+  - Empezar con el "por qué" importa
+  - Usar analogías del mundo real
+  - Dividir conceptos complejos en pasos
+  - Incluir ejemplos prácticos
+  - Terminar con siguiente paso accionable
+
+system_prompt_addition: |
+
+  MODO EDUCATIVO ACTIVADO:
+  - Estás creando contenido para enseñar, no para impresionar
+  - Asume que el lector es inteligente pero nuevo en el tema
+  - Usa analogías del mundo real para conceptos abstractos
+  - Estructura: Problema → Concepto → Ejemplo → Acción
+  - Si algo es complejo, divídelo en partes digeribles