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>

internal/server/http/schedules.go

@@ -30,6 +30,9 @@ type scheduleAPI struct {
 	PreHook         string                `json:"pre_hook,omitempty"`
 	PostHook        string                `json:"post_hook,omitempty"`
 	Enabled         bool                  `json:"enabled"`
+	// Manual = no cron, fires only when the operator triggers a
+	// run-now. Cron expr is ignored when this is true.
+	Manual          bool                  `json:"manual"`
 	CreatedAt       string                `json:"created_at,omitempty"`
 	UpdatedAt       string                `json:"updated_at,omitempty"`
 }
@@ -254,11 +257,13 @@ func validateSchedule(s *scheduleAPI) (code, msg string) {
 	default:
 		return "invalid_kind", "kind must be one of backup|forget|prune|check"
 	}
-	if strings.TrimSpace(s.CronExpr) == "" {
-		return "missing_cron_expr", "cron_expr is required"
-	}
-	if _, err := cronParser.Parse(s.CronExpr); err != nil {
-		return "invalid_cron_expr", err.Error()
+	if !s.Manual {
+		if strings.TrimSpace(s.CronExpr) == "" {
+			return "missing_cron_expr", "cron_expr is required (or set manual=true)"
+		}
+		if _, err := cronParser.Parse(s.CronExpr); err != nil {
+			return "invalid_cron_expr", err.Error()
+		}
 	}
 	if s.Kind == api.JobBackup && len(s.Paths) == 0 {
 		return "missing_paths", "backup schedules require at least one path"
@@ -283,6 +288,7 @@ func toScheduleAPI(s store.Schedule) scheduleAPI {
 		PreHook:         s.PreHook,
 		PostHook:        s.PostHook,
 		Enabled:         s.Enabled,
+		Manual:          s.Manual,
 		CreatedAt:       s.CreatedAt.Format("2006-01-02T15:04:05.999999999Z07:00"),
 		UpdatedAt:       s.UpdatedAt.Format("2006-01-02T15:04:05.999999999Z07:00"),
 	}