feat(deploy): prod OOM swap cushion + edge maintenance page

Two prod-deploy hardening measures (owner-requested):

- Swap file (Ansible common role, swap_size=1G, vm.swappiness=10). The per-container
  memory caps enforce that one service can't eat all RAM, but they overcommit the
  1.9 GiB main host (~2.8 GiB of caps), so a simultaneous spike could hit the kernel
  OOM-killer (and it might pick postgres). A small swap absorbs the overshoot.
  Idempotent, builtin-only (no ansible.posix).

- Edge maintenance page. prod-deploy.sh raises a flag around the rolling swap /
  migration window that the caddy edge serves a static 503 "технические работы" page
  from (deploy/caddy/maintenance.html), for the user-facing routes only — /_gm
  (Grafana) stays reachable. Cleared on any exit (success, health failure + rollback,
  or error) by a shell trap so it can never stick on. The 503 carries Retry-After +
  an X-Scrabble-Maintenance marker so a follow-up SPA overlay can tell a planned
  window apart from a transient error (the static page only catches a fresh load; an
  in-session user needs the app-side overlay). Not zero-downtime — the single
  stateful backend still blips — but the window is graceful instead of raw 502s.

Verified: caddy validate; the gate 503s every non-/_gm path incl. the Connect/gRPC
edge and serves the page + markers; /_gm bypasses; toggling needs no reload
(per-request stat). Ansible --syntax-check + compose config (base+prod) pass.

deploy/caddy/Caddyfile

@@ -33,6 +33,39 @@
 	# is the prod fix. Background + alternatives (incl. serving h3 for real): docs/EDGE_HTTP3.md.
 	header Alt-Svc clear
 
+	# Maintenance gate. While the deploy holds the flag file /srv/maint/on (touched by
+	# prod-deploy.sh around a rolling swap, removed on the script's exit), serve a static
+	# 503 "works in progress" page instead of proxying to a mid-recreate upstream —
+	# a graceful signal rather than raw 502s. Operator surfaces (/_gm) are excluded so
+	# Grafana stays reachable during the window. Caddy stat()s the flag per request, so
+	# toggling it needs no reload; the page + flag live in the caddy config dir bind-mounted
+	# read-only at /srv/maint (docker-compose.yml). The test contour never sets the flag
+	# (only prod-deploy.sh does), so this is inert there.
+	@maintenance {
+		not path /_gm /_gm/*
+		file {
+			root /srv/maint
+			try_files on
+		}
+	}
+	handle @maintenance {
+		error 503
+	}
+	handle_errors {
+		@maint503 expression {err.status_code} == 503
+		handle @maint503 {
+			root * /srv/maint
+			rewrite * /maintenance.html
+			header Retry-After 120
+			# Distinctive maintenance marker so the SPA can tell a planned window apart from
+			# a transient upstream 503 and show its own dimmed overlay (an in-session user
+			# never sees this static page — it only renders on a fresh load). The header
+			# rides every gated response, incl. the Connect/gRPC edge the app polls.
+			header X-Scrabble-Maintenance 1
+			file_server
+		}
+	}
+
 	# Operator surfaces under /_gm: a single shared Basic-Auth, then route.
 	@gm path /_gm /_gm/*
 	handle @gm {