B2: the HTTP upload view failed OPEN when the config entry was
unavailable while the WS path failed closed — both now share one
may_write() policy helper (new auth.py) that denies non-admins when the
policy cannot be read.
B5: _finite now guards room rects, polygon vertices, view_box and
opening coordinates, not just layout positions; the declared
MAX_OPENINGS cap is finally enforced.
L4 (sub-item): every drag pipeline captures the pointer through the
tolerant helper (an inactive pointerId used to kill device/label/resize
drags); decor shapes gained a bounds clamp so they cannot be dragged far
outside the plan and persisted there.
+2 backend tests (16); both changelogs updated in this commit
