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/en/channels.json

@@ -29,6 +29,10 @@
     "channel": "Channel",
     "stored": "Stored",
     "subs": "Subscribers",
+    "lastUpload": "Last upload",
+    "length": "Length",
+    "types": "N/S/L",
+    "typesHint": "Videos by type: Normal / Shorts / Live",
     "sync": "Sync",
     "tags": "Tags",
     "actions": "Actions"
@@ -51,6 +55,7 @@
     "stored": "{{formatted}} stored",
     "subs": "{{formatted}} subs",
     "openOnYouTube": "Open on YouTube",
+    "editTags": "Edit tags",
     "priorityHint": "Your ranking for this channel. Sort the feed by “Channel priority” to bring higher-priority channels to the top.",
     "raisePriority": "Raise priority",
     "lowerPriority": "Lower priority",