refactor: simplify simulation to run forever as paper trade

- No duration limit - runs forever until user stops
- Only 1 running simulation per bot (returns existing if already running)
- Always paper trade (no auto-execute option)
- Removed Pro upgrade banner
- Removed duration and auto-execute config options
- Simplified API to only require token, chain, check_interval

src/backend/app/db/schemas.py

@@ -100,9 +100,7 @@ class BacktestResponse(BaseModel):
 class SimulationCreate(BaseModel):
     token: str
     chain: str
-    duration_seconds: int = 3600
     check_interval: int = 60
-    auto_execute: bool = False
 
     @field_validator("chain")
     @classmethod