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.

internal/server/http/ui_handlers.go

@@ -142,23 +142,21 @@ func (s *Server) handleUIDashboard(w stdhttp.ResponseWriter, r *stdhttp.Request)
 	}
 }
 
-// handleUIRunBackup and handleUIInitRepo are stubbed during the P2
-// redesign data-model rewrite. The dashboard per-host Run-now button
-// is going away (operator clicks into host detail then a per-source-
-// group Run-now), and Init-repo becomes implicit at host enrolment
-// (auto-init dispatched server-side). Phase 4 of the redesign wires
-// the new per-source-group Run-now via /hosts/{id}/source-groups/{gid}/run.
+// Per-host Run-now and manual Init-repo were retired by the P2 redesign.
+// Run-now lives at POST /hosts/{id}/source-groups/{gid}/run; init runs
+// automatically on the agent's first WS connect after enrolment. Both
+// routes return 410 Gone so any cached browser tab gets a clear error.
 
-func (s *Server) handleUIRunBackup(w stdhttp.ResponseWriter, r *stdhttp.Request) {
+func (s *Server) handleUIRunBackupGone(w stdhttp.ResponseWriter, r *stdhttp.Request) {
 	stdhttp.Error(w,
-		"per-host Run-now is being replaced by per-source-group Run-now — see P2 redesign Phase 4",
-		stdhttp.StatusNotImplemented)
+		"per-host Run-now has moved — use POST /hosts/{id}/source-groups/{gid}/run",
+		stdhttp.StatusGone)
 }
 
-func (s *Server) handleUIInitRepo(w stdhttp.ResponseWriter, r *stdhttp.Request) {
+func (s *Server) handleUIInitRepoGone(w stdhttp.ResponseWriter, r *stdhttp.Request) {
 	stdhttp.Error(w,
-		"manual Init-repo is being replaced by auto-init at host enrolment — see P2 redesign Phase 6",
-		stdhttp.StatusNotImplemented)
+		"manual init-repo is gone — the server auto-inits on the agent's first connect",
+		stdhttp.StatusGone)
 }
 
 // addHostPage carries the Add-host form state. The result-state