feat(phase1): Complete development environment configuration

- Configure Alembic for database migrations with initial schema
- Fix base.py to use lazy loading for async engine (avoids import-time issues)
- Change AI model from sonnet to haiku (claude-3-haiku-20240307)
- Fix pytest version compatibility with pytest-asyncio
- Add frontend package-lock.json

Phase 1 tasks completed:
- F1.1: Development environment (Docker, Node.js 20, Python 3.12, venv)
- F1.2: PostgreSQL with 8 categories seeded
- F1.3: Redis connection verified
- F1.4: Anthropic API configured and tested
- F1.5: Backend server + WebSocket verified

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

backend/app/models/base.py

@@ -1,25 +1,32 @@
-from sqlalchemy.ext.declarative import declarative_base
-from sqlalchemy.ext.asyncio import create_async_engine, AsyncSession
-from sqlalchemy.orm import sessionmaker
-from app.config import get_settings
-
-settings = get_settings()
-
-# Convert postgresql:// to postgresql+asyncpg://
-database_url = settings.database_url.replace(
-    "postgresql://", "postgresql+asyncpg://"
-)
-
-engine = create_async_engine(database_url, echo=True)
-
-AsyncSessionLocal = sessionmaker(
-    engine, class_=AsyncSession, expire_on_commit=False
-)
+from sqlalchemy.orm import declarative_base
 
 Base = declarative_base()
 
 
+def get_async_engine():
+    """Create async engine on demand to avoid import-time config issues."""
+    from sqlalchemy.ext.asyncio import create_async_engine
+    from app.config import get_settings
+
+    settings = get_settings()
+    database_url = settings.database_url.replace(
+        "postgresql://", "postgresql+asyncpg://"
+    )
+    return create_async_engine(database_url, echo=True)
+
+
+def get_async_session():
+    """Create async session factory."""
+    from sqlalchemy.ext.asyncio import AsyncSession
+    from sqlalchemy.orm import sessionmaker
+
+    engine = get_async_engine()
+    return sessionmaker(engine, class_=AsyncSession, expire_on_commit=False)
+
+
 async def get_db():
+    """Dependency for FastAPI routes."""
+    AsyncSessionLocal = get_async_session()
     async with AsyncSessionLocal() as session:
         try:
             yield session

backend/app/services/ai_generator.py

@@ -63,7 +63,7 @@ Responde SOLO con el JSON, sin texto adicional."""
 
         try:
             message = self.client.messages.create(
-                model="claude-3-5-sonnet-20241022",
+                model="claude-3-haiku-20240307",
                 max_tokens=2000,
                 messages=[
                     {"role": "user", "content": prompt}