v1.45.1: follow-up review of v1.45.0 — R3-1, R3-2

R3-1 (high): v1.45.0 made the upload safe but left deletion to the client —
after a successful save the card asked the backend to remove everything but the
file it had just committed. Two open editors cannot be ordered: a delayed
request from one deleted the plan the other had just saved, leaving the
accepted configuration pointing at nothing, the exact damage copy-on-write was
introduced to prevent.

houseplan/plan/cleanup is removed. config/set collects inside its own write
lock from the two configurations that bracket the commit (plans.collect_plans):
a file the old revision referenced and the new one does not is superseded and
goes; any other unreferenced upload waits out PLAN_ORPHAN_TTL_S, because a
fresh one may belong to a transaction that has not committed yet. The collector
lives in a pure module so it can be reasoned about and unit-tested without the
HA harness.

R3-2: houseplan-space-card signed its plan url and threw the result away —
getCardSize() mutated a throwaway model while render() rebuilt its own from the
config, so the <image> requested the protected path and got 401 on every
render. Both cards now share ContentSigner (src/signing.ts), which also gives
the static card batching, expiry handling and periodic re-signing.  is
released in finally: one failed request no longer wedges a url for the life of
the page.

Tests: five backend interleaving cases from the report, six unit tests for the
pure collector, smoke_space_card_bg (verified to fail against a v1.45.0 build:
the raw url reaches the DOM and no retry happens). 57 smokes, 124 unit, 22
backend-pure.
Docs: CHANGELOG.md + CHANGELOG.ru.md + ARCHITECTURE.md + TESTING.md + STATUS.md.

docs/TESTING.md

@@ -234,6 +234,18 @@ Run the *core flows* (marked ★ below) in each environment at least once per mi
       are only reachable through /api/houseplan/content/… with a session; the
       old /houseplan_files/plans|files paths return 404 after a restart; old
       stored URLs keep working (rewritten on read) [auto+manual]
+- [ ] Two editors, one plan (v1.45.1, review R3-1): with the same space open in
+      two tabs, attach a background in each in turn — the plan last saved is the
+      one served, and neither commit deletes the other's file. A rejected upload
+      disappears on a later save, not immediately
+      [auto: backend test_late_commit_of_one_client_never_deletes_another_client_s_plan,
+      test_commit_does_not_collect_another_client_s_uncommitted_upload,
+      test_abandoned_uploads_are_collected_once_old]
+- [ ] Static card background (v1.45.1, review R3-2): a houseplan-space-card on a
+      dashboard shows the plan image, not an empty stage; the browser never
+      requests the unsigned path and Home Assistant logs no failed login. A
+      failed signing request is retried on the next render
+      [auto: smoke_space_card_bg]
 - [ ] Rejected save leaves the plan intact (v1.45.0, review R2-1): attach a new
       background, make the config write fail (a second tab saving first is
       enough) — the previously stored plan is still served, with the same or a