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/services/ai/__init__.py

@@ -0,0 +1,24 @@
+"""
+AI Services - Content Generation Engine v2.0
+
+Este módulo contiene los componentes del motor de generación de contenido:
+- PromptLibrary: Carga y renderiza prompts desde YAML
+- ContextEngine: Anti-repetición y selección de best performers
+- ContentGeneratorV2: Interfaz mejorada con DeepSeek
+- PlatformAdapter: Adapta contenido por plataforma
+- ContentValidator: Validación y scoring con IA
+"""
+
+from app.services.ai.prompt_library import PromptLibrary
+from app.services.ai.context_engine import ContextEngine
+from app.services.ai.generator import ContentGeneratorV2
+from app.services.ai.platform_adapter import PlatformAdapter
+from app.services.ai.validator import ContentValidator
+
+__all__ = [
+    "PromptLibrary",
+    "ContextEngine",
+    "ContentGeneratorV2",
+    "PlatformAdapter",
+    "ContentValidator",
+]