cuprated: Add graceful shutdown, pt3: Exit codes and main error propagation

[redsh4de]

What

Depends on #586

Building on part 2, this replaces hard exit and panics across cuprated's startup path with error propagation, introduces ExitCode to distinguish clean shutdowns from failures.

Based off #528
Closes #522.

Where

• cuprated
  • main.rs - main() returns ExitCode, new main_inner() propagates all startup errors via ?, removed all process::exit(1) calls from config/init path
  • monitor.rs - TaskExecutor now has failed: Arc<AtomicBool> flag, spawn_critical sets it on error, has_failed() function for exit code decision

How

main_inner() returns anyhow::Result<ExitCode>. Startup errors propagate to main(), which prints them with eprintln_red and returns ExitCode::FAILURE.

For runtime errors, spawn_critical sets an AtomicBool flag on the TaskExecutor before triggering shutdown. After the runtime is dropped, main() reads has_failed() to decide between ExitCode::SUCCESS and ExitCode::FAILURE.