v1.47.0: pick a plan you already uploaded

Closes both findings from the v1.46.6 review with one feature, because they are
the same gap seen from two sides. HP-1466-02: a detached plan stayed on disk and
could not be re-attached from the card — the old url is nowhere in the config,
and the backend test 'proved' reattach by remembering it in a Python variable.
HP-1466-01: files kept forever with no way to see or remove them is not a
policy, it is accumulation.

New: houseplan/plans/list (name, url, size, modified, and which spaces use it)
and houseplan/plans/delete, which refuses while a space still references the
file — the stored configuration answers that, not the client. In the space
dialog, 'Already uploaded' shows the list with thumbnails; one click attaches,
reading the aspect from the image as an upload does; the trash button is the
only way a plan file is ever deleted.

That also bounds the disk without any timer, which is the part every automatic
attempt got wrong: v1.46.4 deleted detached plans, v1.46.5 raced the retry that
was about to reference an upload. The user decides, and can now see what they
are deciding about.

Docs: comments in plans.py and websocket_api.py still described the age-based
collection v1.46.6 removed (report §6); ARCHITECTURE gained the two new routes
and an explanation of why the listing is what makes 'never delete' livable.

docs/TESTING.md

@@ -239,6 +239,12 @@ Run the *core flows* (marked ★ below) in each environment at least once per mi
       on the plan after a reload. Same for each tap action and each fill mode
       [auto: backend test_every_display_mode_the_editor_offers_is_accepted and
       neighbours, test_a_marker_showing_its_value_can_be_saved]
+- [ ] Re-attaching a detached plan (v1.47.0): detach a plan, save, RELOAD THE
+      PAGE, open space settings → "Already uploaded" → the image is listed with
+      its size and no "in use" note → attach it → it renders. The one a space
+      uses shows that space and cannot be deleted; a free one can, with a
+      confirm, and disappears from the list
+      [auto: smoke_saved_plans + backend test_stored_plans_can_be_listed_and_deleted_on_request]
 - [ ] Detaching a plan keeps the file (v1.46.6): switch a space to "draw" and
       SAVE — the image is still in `config/houseplan/plans/` right afterwards,
       and after a restart, and can be re-attached. Deleting the space keeps it