feat(media): delete videos from the media library

Add an admin flow to permanently delete a media asset and clean up
everything it leaves behind.

- Delete dialog with a destructive, irreversible confirmation; deletion
  is blocked while the video is still attached and shows the in-use
  lessons so the admin can detach first.
- Usage list resolves lesson/course context for each attachment, links
  to the lesson editor via an anchor (right-click "open in new tab"),
  and exposes a refresh button to re-check usage after detaching
  elsewhere.
- Storage cleanup runs as a background maintenance job: the producer
  captures the asset's object keys/prefixes up front and an idempotent
  worker sweeps HLS segments, thumbnails, audio, transcript, and the
  raw upload from object storage.

Also harden usage accounting so stale attachments cannot trap an asset:

- asset_usages has no FK to its polymorphic target, so prune a lesson's
  usage rows whenever the lesson is hard-deleted (single + bulk paths).
- Usage resolution drops orphaned lesson usages (lesson gone or course
  soft-deleted) so they neither appear in the list nor block deletion.

Document the icon-only button convention (variant="secondary") in
AGENTS.md.

apps/dashboard/src/lib/utils/translations/en.json

@@ -1081,7 +1081,18 @@
       "edit": "Edit",
       "where_used": "Where used",
       "download": "Download",
-      "manage_thumbnails": "Manage thumbnails"
+      "manage_thumbnails": "Manage thumbnails",
+      "delete": "Delete"
+    },
+    "delete": {
+      "title": "Delete video",
+      "irreversible_title": "This cannot be undone",
+      "irreversible_description": "This video and all of its files — renditions, thumbnails, and captions — will be permanently deleted and cannot be recovered.",
+      "in_use_title": "Still in use",
+      "in_use_description": "This video is attached to the lessons below. Open each lesson, remove the video there, then come back to delete it.",
+      "confirm": "Delete permanently",
+      "cancel": "Cancel",
+      "close": "Close"
     },
     "pagination": {
       "previous": "Previous",
@@ -1128,12 +1139,19 @@
       "loading": "Loading usage details...",
       "empty": "This asset is not attached anywhere yet.",
       "total": "Total usages:",
+      "go_to_lesson": "Go to lesson",
+      "refresh": "Refresh usage",
       "target": {
         "lesson": "Lesson",
         "exercise": "Exercise",
         "question": "Question",
         "unknown": "Unknown"
       },
+      "slot_type": {
+        "lesson_video": "Lesson video",
+        "lesson_document": "Lesson document",
+        "unknown": "Attachment"
+      },
       "table": {
         "target_type": "Target Type",
         "target_id": "Target ID",
@@ -4121,6 +4139,9 @@
       "update_failed": "Failed to update media asset",
       "update_success": "Media asset updated",
       "usage_failed": "Failed to load asset usage",
+      "delete_success": "Video deleted",
+      "delete_in_use": "This video is still attached to a lesson. Remove it there first.",
+      "delete_failed": "Failed to delete video",
       "download_failed": "Failed to get asset download URL",
       "thumbnail_uploaded": "Thumbnail uploaded",
       "thumbnail_upload_failed": "Failed to upload thumbnail",