Add bbox coordinates to object_pictures for chip highlight feature

- Add bbox_x/y/w/h FLOAT columns to object_pictures (0–1 percentage range)
- Include type ('word'|'object') and bbox in feed placeholder response
- Fix picture query to use DISTINCT ON instead of LIMIT 1

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/db-migrate.js

@@ -310,6 +310,12 @@ async function migrate() {
     )
   `);
 
+  // Bounding-box columns for object highlights (percentage 0–1 of image size)
+  await query(`ALTER TABLE object_pictures ADD COLUMN IF NOT EXISTS bbox_x FLOAT`);
+  await query(`ALTER TABLE object_pictures ADD COLUMN IF NOT EXISTS bbox_y FLOAT`);
+  await query(`ALTER TABLE object_pictures ADD COLUMN IF NOT EXISTS bbox_w FLOAT`);
+  await query(`ALTER TABLE object_pictures ADD COLUMN IF NOT EXISTS bbox_h FLOAT`);
+
   // M2M: objects <-> pairs (Platzhalter)
   await query(`
     CREATE TABLE IF NOT EXISTS object_pairs (