feat: backend project setup with FastAPI structure and dependencies

- Create directory structure per IMPLEMENTATION_PLAN.md Section 12
- Add requirements.txt with FastAPI, SQLAlchemy, CrewAI, etc.
- Add core/config.py for environment variable configuration
- Add core/database.py for SQLite connection
- Add core/security.py for password hashing and JWT
- Add FastAPI app entry point (main.py) with all API routers
- Add Uvicorn runner (run.py)
- Add API route stubs (auth, bots, backtest, simulate, config)
- Add db/models.py with SQLAlchemy models
- Add db/schemas.py with Pydantic schemas
- Add service stubs (ai_agent, backtest, simulate engines)
- Add .env.example with all required environment variables
- Verify server starts correctly

src/backend/app/core/database.py

@@ -0,0 +1,40 @@
+import os
+from sqlalchemy import create_engine, event
+from sqlalchemy.orm import sessionmaker, declarative_base
+from sqlalchemy.engine import Engine
+from .config import get_settings
+
+settings = get_settings()
+
+if settings.DATABASE_URL.startswith("sqlite"):
+    db_path = settings.DATABASE_URL.replace("sqlite:///", "")
+    os.makedirs(
+        os.path.dirname(db_path) if os.path.dirname(db_path) else ".", exist_ok=True
+    )
+
+    @event.listens_for(Engine, "connect")
+    def set_sqlite_pragma(dbapi_conn, connection_record):
+        cursor = dbapi_conn.cursor()
+        cursor.execute("PRAGMA foreign_keys=ON")
+        cursor.close()
+
+
+engine = create_engine(
+    settings.DATABASE_URL,
+    connect_args={"check_same_thread": False}
+    if settings.DATABASE_URL.startswith("sqlite")
+    else {},
+    echo=settings.DEBUG,
+)
+
+SessionLocal = sessionmaker(autocommit=False, autoflush=False, bind=engine)
+
+Base = declarative_base()
+
+
+def get_db():
+    db = SessionLocal()
+    try:
+        yield db
+    finally:
+        db.close()