Photo lightbox, timeline photo previews, subtle edit/delete footer

- Photos open in a full-screen lightbox overlay instead of navigating to a new tab (fixes PWA back-button issue)
- DB migration adds photo_id to task_events; photo upload logs the reference so activity timeline can show a clickable thumbnail for each 'Photo added' event
- Edit and Delete buttons moved to a quiet ghost-button footer at the bottom-right of the modal

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

backend/src/routes/photos.js

@@ -57,7 +57,7 @@ export async function photoRoutes(app, opts) {
        VALUES ($1,$2,$3,$4,$5,$6,$7) RETURNING *`,
       [taskId, fileData.originalName, filePath, 'image/jpeg', fileData.size, stage, req.user.email]
     )
-    await logEvent(taskId, 'photo', { note: `Photo added (${stage})`, userName: req.user.name })
+    await logEvent(taskId, 'photo', { note: `Photo added (${stage})`, userName: req.user.name, photoId: ins[0].id })
     return ins[0]
   })