feat(files): share a folder by link — its page lists what is in it, plays a video above the list

A /watch link may now open one of its owner's own folders (migration 0108: folder_id beside job_id,
exactly one set; a folder's links go with it). The page shows what is in the folder when it is
opened, newest first: a video or a song plays in the same player a file link has, above the list,
each keeping its own resume point; a document only downloads, never opens as a page. A file moved
out stops opening through the link at its next request. Password, expiry and stream-only work as on
a file link; the unfurl names the folder and counts its files, never their names. Source folders
and the Hub cannot be shared.

The folder menu gets "Copy link" and "Share with a link…" (the Share dialog's link half); the rail
marks a folder a live link opens.

Two fixes in the Share dialog on the way: revoking the last link no longer mints a new one at once,
and the new-link form goes back to downloadable after making a link.

frontend/src/lib/api/files.ts

@@ -18,6 +18,8 @@ export interface FileFolder {
   count: number; // its done files
   size_bytes: number;
   created_at: string | null;
+  // A live public link opens it: whatever is moved in is shown to whoever holds the link.
+  has_link: boolean;
 }
 
 // The most files one list answers with (backend `routes/files.py` MAX_LIST, held equal by