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/da.json

@@ -357,7 +357,11 @@
       "thumbnail_regen_failed": "Kunne ikke genskabe miniaturebilleder",
       "transcript_saved": "Udskrift opdateret",
       "transcript_save_failed": "Kunne ikke gemme transskription",
-      "transcription_already_running": "Et transskriptionsjob er allerede i gang for denne video"
+      "transcription_already_running": "Et transskriptionsjob er allerede i gang for denne video",
+      "video_added_to_lesson": "Video tilføjet til lektion",
+      "delete_success": "Video slettet",
+      "delete_in_use": "Denne video er stadig knyttet til en lektion. Fjern det der først.",
+      "delete_failed": "Videoen kunne ikke slettes"
     },
     "tags": {
       "group_created": "Taggruppe oprettet",
@@ -2808,7 +2812,14 @@
         "exercise": "Motion",
         "question": "Spørgsmål",
         "unknown": "Ukendt"
-      }
+      },
+      "go_to_lesson": "Gå til lektion",
+      "slot_type": {
+        "lesson_video": "Lektion video",
+        "lesson_document": "Lektionsdokument",
+        "unknown": "Vedhæftet fil"
+      },
+      "refresh": "Opdater brugen"
     },
     "form": {
       "file_name": "Filnavn",
@@ -2838,7 +2849,8 @@
       "edit": "Rediger",
       "where_used": "Hvor brugt",
       "download": "Download",
-      "manage_thumbnails": "Administrer thumbnails"
+      "manage_thumbnails": "Administrer thumbnails",
+      "delete": "Slet"
     },
     "table": {
       "title": "Titel",
@@ -2910,6 +2922,16 @@
       "custom_preview_alt": "Forhåndsvisning af tilpasset miniaturebillede",
       "regenerate": "Regenerer",
       "save_selection": "Gem thumbnail"
+    },
+    "delete": {
+      "title": "Slet video",
+      "irreversible_title": "Dette kan ikke fortrydes",
+      "irreversible_description": "Denne video og alle dens filer – gengivelser, miniaturebilleder og billedtekster – slettes permanent og kan ikke gendannes.",
+      "in_use_title": "Stadig i brug",
+      "in_use_description": "Denne video er vedhæftet nedenstående lektioner. Åbn hver lektion, fjern videoen der, og kom så tilbage for at slette den.",
+      "confirm": "Slet permanent",
+      "cancel": "Annuller",
+      "close": "Luk"
     }
   },
   "settings": {