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/alembic/versions/0108_folder_links.py

@@ -0,0 +1,40 @@
+"""A share link may open a folder
+
+Revision ID: 0108_folder_links
+Revises: 0107_link_token_at_rest
+Create Date: 2026-10-15
+
+A `/watch` link opened one file. Now it may open one of its owner's own folders instead: the link's
+page lists what is in the folder when it is opened. `folder_id` sits beside `job_id`, exactly one of
+the two set; a folder's links go with the folder.
+
+The downgrade has nowhere to keep a folder link, so it deletes them: they stop opening.
+"""
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0108_folder_links"
+down_revision: str | None = "0107_link_token_at_rest"
+branch_labels: str | None = None
+depends_on: str | None = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "download_links",
+        sa.Column("folder_id", sa.Integer(), sa.ForeignKey("file_folders.id", ondelete="CASCADE")),
+    )
+    op.create_index("ix_download_links_folder_id", "download_links", ["folder_id"])
+    op.alter_column("download_links", "job_id", nullable=True)
+    op.create_check_constraint(
+        "ck_download_link_target", "download_links", "(job_id IS NULL) <> (folder_id IS NULL)"
+    )
+
+
+def downgrade() -> None:
+    op.execute("DELETE FROM download_links WHERE folder_id IS NOT NULL")
+    op.drop_constraint("ck_download_link_target", "download_links", type_="check")
+    op.alter_column("download_links", "job_id", nullable=False)
+    op.drop_index("ix_download_links_folder_id", table_name="download_links")
+    op.drop_column("download_links", "folder_id")