fix: replace per-element CASE WHEN in migration 003 with WHERE EXISTS

Instead of applying a per-element heuristic (skip if value ≤ 50), identify
pixel-scale rooms at the row level with WHERE EXISTS, then convert all
elements unconditionally. Eliminates the risk of mixed-scale elements within
the same room.

backend/migrations/003_normalize_room_layout_units.sql

@@ -1,30 +1,26 @@
 -- Normalize room layout units: divide pixel-scale coordinates by 40.
--- Applied per element with CASE WHEN so the transform is idempotent:
--- coordinates already in grid units (≤50) are left untouched.
--- PRECONDITION: assumes no pre-existing room has pixel-scale coords ≤50.
--- Any element with pixel x/y/width/height ≤50 will be skipped (treated as
--- already converted). Verify this holds before running on a production DB.
+-- Only runs on rooms that contain at least one element with a coordinate > 50
+-- (i.e. still in pixel scale). Once identified, all elements are converted
+-- unconditionally — no per-element heuristic needed.
 UPDATE rooms
 SET layout_json = (
     SELECT json_group_array(
         json_object(
             'id', json_extract(value, '$.id'),
             'label', json_extract(value, '$.label'),
-            'x', CASE WHEN CAST(json_extract(value, '$.x') AS REAL) > 50
-                      THEN ROUND(CAST(json_extract(value, '$.x') AS REAL) / 40.0, 2)
-                      ELSE json_extract(value, '$.x') END,
-            'y', CASE WHEN CAST(json_extract(value, '$.y') AS REAL) > 50
-                      THEN ROUND(CAST(json_extract(value, '$.y') AS REAL) / 40.0, 2)
-                      ELSE json_extract(value, '$.y') END,
-            'width', CASE WHEN CAST(json_extract(value, '$.width') AS REAL) > 50
-                         THEN ROUND(CAST(json_extract(value, '$.width') AS REAL) / 40.0, 2)
-                         ELSE json_extract(value, '$.width') END,
-            'height', CASE WHEN CAST(json_extract(value, '$.height') AS REAL) > 50
-                          THEN ROUND(CAST(json_extract(value, '$.height') AS REAL) / 40.0, 2)
-                          ELSE json_extract(value, '$.height') END,
+            'x', ROUND(CAST(json_extract(value, '$.x') AS REAL) / 40.0, 2),
+            'y', ROUND(CAST(json_extract(value, '$.y') AS REAL) / 40.0, 2),
+            'width', ROUND(CAST(json_extract(value, '$.width') AS REAL) / 40.0, 2),
+            'height', ROUND(CAST(json_extract(value, '$.height') AS REAL) / 40.0, 2),
             'type', json_extract(value, '$.type')
         )
     )
     FROM json_each(rooms.layout_json)
 )
-WHERE json_array_length(layout_json) > 0;
+WHERE EXISTS (
+    SELECT 1 FROM json_each(rooms.layout_json)
+    WHERE json_extract(value, '$.x') > 50
+       OR json_extract(value, '$.y') > 50
+       OR json_extract(value, '$.width') > 50
+       OR json_extract(value, '$.height') > 50
+);