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.

backend/app/routes/files.py

@@ -47,6 +47,7 @@ from app.mailtext import lang_for_user
 from app.models import (
     DISPLAY_TEXT_MAX,
     DownloadJob,
+    DownloadLink,
     DownloadShare,
     FileFolder,
     LibraryState,
@@ -352,7 +353,7 @@ def readable_job(db: Session, caller: FilesCaller, job_id: int) -> DownloadJob:
 # --- folders -------------------------------------------------------------------------------
 
 
-def _serialize_folder(folder: FileFolder, count: int, size: int) -> dict:
+def _serialize_folder(folder: FileFolder, count: int, size: int, linked: bool = False) -> dict:
     return {
         "id": folder.id,
         "kind": folder.kind,
@@ -363,9 +364,23 @@ def _serialize_folder(folder: FileFolder, count: int, size: int) -> dict:
         "count": count,
         "size_bytes": size,
         "created_at": iso(folder.created_at),
+        # A live public link opens it: whatever is moved in is shown to whoever holds the link.
+        "has_link": linked,
     }
 
 
+def _linked(db: Session, folder_ids: list[int]) -> set[int]:
+    """Which of these folders a live public link opens (one not past its expiry)."""
+    now = datetime.now(timezone.utc)
+    rows = db.execute(
+        select(DownloadLink.folder_id).where(
+            DownloadLink.folder_id.in_(folder_ids),
+            or_(DownloadLink.expires_at.is_(None), DownloadLink.expires_at > now),
+        )
+    ).scalars()
+    return {folder_id for folder_id in rows if folder_id is not None}
+
+
 @router.get("/folders")
 def list_folders(
     caller: FilesCaller = Depends(files_caller), db: Session = Depends(get_db)
@@ -388,7 +403,8 @@ def list_folders(
         .group_by(FileFolder.id)
         .order_by(FileFolder.kind.desc(), FileFolder.source_key, FileFolder.name, FileFolder.id)
     ).all()
-    return [_serialize_folder(f, count, int(size)) for f, count, size in rows]
+    linked = _linked(db, [f.id for f, _, _ in rows])
+    return [_serialize_folder(f, count, int(size), f.id in linked) for f, count, size in rows]
 
 
 class FolderIn(BaseModel):
@@ -471,7 +487,7 @@ def update_folder(
             )
     db.commit()
     count, size = _folder_totals(db, folder.id)
-    return _serialize_folder(folder, count, size)
+    return _serialize_folder(folder, count, size, folder.id in _linked(db, [folder.id]))
 
 
 def _folder_totals(db: Session, folder_id: int) -> tuple[int, int]: