feat(channels): aggregate columns + smarter tag display

- Add Last upload, total Length and a Normal/Shorts/Live breakdown, computed on read in the
  existing grouped query (~35ms over the full catalog — measured, so no denormalization).
- Subscribers shown compact (919K), since the YouTube API already rounds them.
- Tag cell now shows only the tags actually attached to a channel; a '+' opens a per-channel
  picker to attach/detach, instead of listing every user tag on every row.

frontend/src/i18n/locales/de/channels.json

@@ -29,6 +29,10 @@
     "channel": "Kanal",
     "stored": "Gespeichert",
     "subs": "Abonnenten",
+    "lastUpload": "Letzter Upload",
+    "length": "Länge",
+    "types": "N/S/L",
+    "typesHint": "Videos nach Typ: Normale / Shorts / Live",
     "sync": "Sync",
     "tags": "Tags",
     "actions": "Aktionen"
@@ -51,6 +55,7 @@
     "stored": "{{formatted}} gespeichert",
     "subs": "{{formatted}} Abonnenten",
     "openOnYouTube": "Auf YouTube öffnen",
+    "editTags": "Tags bearbeiten",
     "priorityHint": "Deine Einstufung für diesen Kanal. Sortiere den Feed nach „Kanalpriorität“, um Kanäle mit höherer Priorität nach oben zu bringen.",
     "raisePriority": "Priorität erhöhen",
     "lowerPriority": "Priorität senken",