P2-04.5: kill host.default_paths in favour of manual schedules

Two independent path lists for "what does this host back up?" was
a real divergence footgun — operator types one set at Add-host time
and a different set into a schedule, both end up in the same repo,
the snapshot history looks fine until restore. Resolution: drop
host.default_paths entirely; add a `manual` flag on schedules.
A manual schedule has paths/excludes/tags/retention like any other
but no cron — it fires only via per-schedule Run-now. Single source
of truth for what gets backed up.

Schema (migration 0007):
* schedules.manual INTEGER NOT NULL DEFAULT 0.
* For every host with non-empty default_paths, seed a manual
  schedule with those paths and bump host_schedule_version.
* ALTER TABLE hosts DROP COLUMN default_paths.
* ALTER TABLE enrollment_tokens RENAME COLUMN default_paths
  TO initial_paths.

Original draft of this migration rebuilt hosts via the
create-new + drop-old + rename-new pattern. With foreign_keys=ON
(set in the connection DSN), DROP TABLE on the parent fired
ON DELETE CASCADE on every child of hosts(id) — schedules /
jobs / snapshots / host_credentials all wiped on the smoke env
when I tried it. SQLite 3.35+ supports column-level ALTERs
directly, so we skip the rebuild dance and avoid the cascade
trap. Six lines of SQL instead of sixty, no FK risk.

Run-now rewiring:
* New `dispatchScheduleNow(hostID, scheduleID, conn?)` helper
  unifies the agent-driven path (cron fire → schedule.fire →
  OnScheduleFire callback) and the UI-driven path (operator
  clicks Run-now on a schedule row). Conn arg is optional; nil
  falls back to Hub.Send.
* New POST /hosts/{id}/schedules/{sid}/run endpoint — per-row
  Run-now button on the schedules list.
* Dashboard's per-host Run-now (handleUIRunBackup) now picks the
  host's only enabled manual schedule, falls back to the only
  enabled schedule, else returns "pick one in Schedules tab".
  Keeps one-click for the common case.

Agent:
* Scheduler skips manual schedules in cron build (silent — they're
  a normal data shape, not an error).
* Wire Schedule struct gains Manual flag.
* Schedule.fire flow unchanged — the agent only ever fires
  non-manual schedules anyway.

UI:
* Add-host form retitled "Initial schedule · manual" so the
  operator knows the paths become an editable schedule under
  the Schedules tab. Result page calls out the manual schedule
  + points at Host > Schedules.
* Schedule edit form: "Manual schedule" checkbox at the top of
  the When section; toggling it hides/shows the cron field via
  inline JS. Server-side validator skips the cron requirement
  when manual=true.
* Schedule list shows a "manual" tag under the status pill and
  renders the When column as "— run-now only —" for manual rows.
  Each row gets a Run-now button when the schedule is enabled
  and the host is online.

Tests + go test ./... green.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/templates/pages/schedule_edit.html

@@ -38,9 +38,18 @@
     <div class="col-span-7 panel rounded-[7px] px-8 py-7">
 
       <h3 class="text-[13px] font-semibold uppercase tracking-[0.08em] text-ink-mute mb-4">When</h3>
+
       <div class="mb-5">
+        <label class="flex items-center gap-2.5 cursor-pointer text-[13px]">
+          <input id="se-manual" type="checkbox" name="manual" {{if $page.Manual}}checked{{end}}
+                 onchange="document.getElementById('se-cron-block').style.display=this.checked?'none':'';">
+          <span>Manual schedule <span class="text-ink-fade font-normal">— no cron, only fires when you click Run-now</span></span>
+        </label>
+      </div>
+
+      <div id="se-cron-block" class="mb-5" {{if $page.Manual}}style="display: none;"{{end}}>
         <label class="field-label" for="se-cron">Cron expression</label>
-        <input id="se-cron" name="cron_expr" type="text" class="field mono" required value="{{$page.CronExpr}}">
+        <input id="se-cron" name="cron_expr" type="text" class="field mono" value="{{$page.CronExpr}}">
         <div class="field-help">
           Standard 5-field cron with descriptors. Examples:
           <span class="mono text-ink-mid">0 3 * * *</span> (daily 03:00),