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

frontend/packages/dashboard/src/system-sections.css

@@ -122,34 +122,6 @@
   color: var(--amber);
   font-weight: bold;
 }
-.rqe-source {
-  font-size: 0.75em;
-  padding: 0.05em 0.45em;
-  border-radius: 0.7em;
-  border: 1px solid var(--border);
-  color: var(--muted);
-  text-transform: uppercase;
-  letter-spacing: 0.05em;
-}
-.rqe-source-manual {
-  color: var(--cyan);
-  border-color: var(--cyan);
-}
-.rqe-source-meta_update {
-  color: var(--purple);
-  border-color: var(--purple);
-}
-.rqe-source-auto_update {
-  color: var(--muted);
-}
-.rqe-source-crash_recover {
-  color: var(--amber);
-  border-color: var(--amber);
-}
-.rqe-source-approval {
-  color: var(--green);
-  border-color: var(--green);
-}
 /* ─── running-rebuild live log (renderRebuildLiveLog in builds.js) ──────────
    Streams the currently-running rebuild's build log inline under the queue.
    Hidden (native `hidden` attr) when nothing is building. Used on