hive: remove the read-only schedules page and the old schedule store

Scheduled prompts live at swarm level (swarm-controller's
/api/schedules, `swarmctl schedule`, the swarm UI's /schedules page).
The hive dashboard's SCH3DUL3S tab only listed the rows left in
hive-c0re's `scheduled_prompts` table so operators could copy them
over; the operator confirmed everything is migrated.

Removed:
- the SCH3DUL3S tab, its pane, schedules.js, its CSS (including the
  `.rqe-source*` rules, whose only emitter was schedules.js) and the
  hub tile's "schedules" entry
- hive-c0re's GET /api/schedules and the `scheduled_prompts` store
  (with its v1 column migration and test)
- hive-sh4re's `schedule` wire types, which only that route used
- docs describing the tab, the route and the two tables

The rebuild-queue cancel route that shared the module moves to
dashboard/rebuild_queue.rs under an OpenAPI tag of its own.

The two sqlite tables are not dropped: nothing opens them any more,
and persistence.md says they can be dropped by hand.

Closes #4916

hive-c0re/src/coordinator.rs

@@ -29,10 +29,6 @@ const LAST_STOPPED_RUNNING_KEY: &str = "last_stopped_running";
 
 pub struct Coordinator {
     pub broker: Arc<Broker>,
-    /// Scheduled-prompt rows, read-only: the dashboard lists them so the
-    /// operator can recreate them with `swarmctl schedule`. Nothing on
-    /// the hive writes or fires them.
-    pub scheduled_prompts: Arc<crate::scheduled_prompts::ScheduledPrompts>,
     /// Full build-log capture. `lifecycle::run` /
     /// `lifecycle::prebuild_toplevel` `start()` a row per attempt,
     /// pipe every stdout/stderr line into it, and `finish()` it on
@@ -452,8 +448,6 @@ impl Coordinator {
             agent_io_weight,
         } = env;
         let broker = Broker::open(db_path).context("open broker")?;
-        let scheduled_prompts = crate::scheduled_prompts::ScheduledPrompts::open(db_path)
-            .context("open scheduled_prompts")?;
         // BuildLogs wants a directory (it picks its own `build_logs.sqlite`
         // file under it); every other opener here takes the sibling
         // sqlite-file path itself. Derive the dir from `db_path`'s
@@ -481,7 +475,6 @@ impl Coordinator {
         let (shutdown_tx, _) = watch::channel(false);
         Ok(Self {
             broker: Arc::new(broker),
-            scheduled_prompts: Arc::new(scheduled_prompts),
             build_logs,
             hyperhive_flake,
             hyperhive_docs_flake,