Add support for --loopback-no-mix flag and topology synchronization

- Introduced the `--loopback-no-mix` flag to control 2-node vs 4-node loopback topologies.
- Updated topology handling logic to recompute and migrate as needed based on loopback flags.
- Enhanced database schema to include `loopback_no_mix` with automatic migration for backward compatibility.
- Refactored `create`, `edit`, and `route` commands to integrate new topology management capabilities.
- Improved `list` command output to include architecture details.

src/cli.rs

@@ -74,6 +74,11 @@ pub struct EditArgs {
     #[arg(short, long, value_name = "BOOL")]
     pub loopback: Option<bool>,
 
+    /// Keep a mixed-in source audible in the loopback instead of isolating
+    /// it (2-node topology instead of 4).
+    #[arg(long = "loopback-no-mix", value_name = "BOOL")]
+    pub loopback_no_mix: Option<bool>,
+
     /// Loopback volume: fraction (0.8) or percent (80).
     #[arg(short, long, value_name = "PCT")]
     pub volume: Option<f32>,