#20 Update appview CLI structure to match opake-cli command pattern

Pattern to match

CLI pattern:

• commands/mod.rs — declares submodules + Execute trait
• Each command in its own file: commands/foo.rs with #[derive(Args)] pub struct FooCommand
• main.rs — Cli struct with #[command(subcommand)], match dispatch

Current appview state:

• Everything in main.rs — Cli, Command enum, and all run_*/status/serve functions inline
• No commands/ directory

Plan: 1. Create commands/mod.rs with subcommand enum + dispatch 2. Create commands/run.rs, commands/index.rs, commands/serve.rs, commands/status.rs 3. Move the run_full, run_index, run_serve, run_status bodies into command structs 4. Keep shared helpers (load_config_or_exit, open_db_or_exit, build_state, serve_http, shutdown_signal, spawn_indexer) in main.rs — they're infra, not commands 5. Slim main.rs to parse + dispatch