### Context & Goal

Currently, every command manually performs checks like user existence or maintenance mode, or these are hardcoded into the `CommandHandler`. Standardizing these requirements in the command definition itself makes the code cleaner and more declarative.

### Dependencies

- None

### Affected Files

- `shared/lib/types.ts`: Update `Command` interface to include a optional `requirements` object.
- `bot/lib/handlers/CommandHandler.ts`: Update to read and enforce these requirements.
- `bot/commands/economy/balance.ts`: Refactor to use the new requirements (example).

### Technical Constraints & Strategy

- Implementation: Use a standardized `requirements` object in the `Command` interface.
- Requirements could include: `userExists: boolean`, `permissions: string[]`, `devOnly: boolean`.
- Ensure `CommandHandler` provides clear error messages to the user when a requirement fails.

### Definition of Done (Binary)

- [ ] `Command` interface updated in `types.ts`.
- [ ] `CommandHandler.ts` enforces requirements before executing command.
- [ ] At least one command (e.g., `balance`) is refactored to use the new system.
- [ ] Clear error embeds are shown to the user when requirements aren't met.

### New Test Files

- None (Verification via manual testing of command execution).