P2R-01: REST + WS rewire against the slim shape

Schedules CRUD now takes {cron, enabled, source_group_ids[]} with cron
parsed via robfig/cron/v3 and group membership scoped to the host.
New source-groups CRUD lives at /api/hosts/{id}/source-groups; delete
refuses with 409 if any schedule still references the group, returning
the schedule list so the UI can prompt 'remove from these schedules
first.' Repo-maintenance GET/PUT manages forget/prune/check cadences
on host_repo_maintenance — no version bump, the server-side ticker
(P2R-06) drives execution.

Per-source-group Run-now (POST /hosts/{id}/source-groups/{gid}/run)
resolves the group's includes/excludes/retention/tag and dispatches a
backup command.run with the new structured CommandRunPayload fields
(Includes/Excludes/Tag). Old per-host /hosts/{id}/run-backup and
/hosts/{id}/init-repo return 410 Gone with a redirect message.

schedule_push.go is rebuilt: buildScheduleSetPayload assembles the
slim wire shape, pushScheduleSetOnConn ships it during the on-hello
window, pushScheduleSetAsync fires after every CRUD mutation, and
dispatchScheduledJob handles agent schedule.fire by iterating the
schedule's source groups and dispatching one backup per group with
actor_kind=schedule and scheduled_id pointing at the schedule.

Auto-init at first WS connect: when the host has repo creds bound and
no init job in its history, server dispatches restic init. Restic's
'config file already exists' soft-success means re-runs against an
existing repo no-op; we don't auto-retry on failure (operator triggers
re-init manually via the danger zone in P2R-09).

api.Schedule drops Kind/Paths/Excludes/Tags/RetentionPolicy/Manual etc.
in favour of {id, cron, enabled, source_groups: [...]}. The agent
scheduler stops checking sch.Manual; cmd/agent's backup dispatch reads
Includes/Excludes/Tag instead of Args.

Tests cover the new HTTP surface end-to-end: source-groups CRUD with
in-use refusal, schedule validation (bad cron / missing groups /
foreign group), repo-maintenance auto-seed and validation, the 410
route, and buildScheduleSetPayload's wire-shape correctness. Full
suite passes; smoke env exercises auto-init dispatch on hello,
async push after schedule create, and per-source-group Run-now
landing the right paths/excludes/tag at the agent.

cmd/agent/main.go

@@ -260,12 +260,22 @@ func (d *dispatcher) runJob(ctx context.Context, p api.CommandRunPayload, tx wsc
 
 	switch p.Kind {
 	case api.JobBackup:
-		// Agent.Args carries [paths...]. Excludes/tags are not yet
-		// surfaced over the wire; they come with P2 schedule support.
+		// Includes/Excludes/Tag come from the source group resolved
+		// server-side. Args is preserved for backwards compatibility:
+		// if the server sends only Args (older shape) we fall back to
+		// treating it as the paths list with no tag.
+		paths := p.Includes
+		if len(paths) == 0 {
+			paths = p.Args
+		}
+		var tags []string
+		if p.Tag != "" {
+			tags = []string{p.Tag}
+		}
 		slog.Info("agent: accepting backup job",
-			"job_id", p.JobID, "paths", p.Args)
+			"job_id", p.JobID, "paths", paths, "excludes", p.Excludes, "tag", p.Tag)
 		go func() {
-			if err := r.RunBackup(ctx, p.JobID, p.Args, nil, nil); err != nil {
+			if err := r.RunBackup(ctx, p.JobID, paths, p.Excludes, tags); err != nil {
 				slog.Warn("agent: backup job failed", "job_id", p.JobID, "err", err)
 				return
 			}