feat(files): the Files page — the library in one aligned list, its folders in a rail

Files is the one library: every done file, the ones shared with me, or one
folder's, picked in a side rail (sources, own folders, the storage meter) and
mirrored to ?folder=. One table with every kind of value in its own column;
Copy link, Share and Save in sight, every action on a right click: play,
cut, rename in the row, move to an own folder or back home, keep, details,
re-download, delete.

The Downloads page keeps only the queue until the tray replaces it. The old
/api/downloads library routes are gone: list shared, upload, the details
PATCH, delete, the file and the usage now live under /api/files, and a
listed file says which source it came from. The job's "expired" flag is gone
with the state it described.

backend/tests/test_upload_conversion.py

@@ -549,7 +549,7 @@ def test_a_link_says_preparing_only_while_its_video_is_being_made(
     _probe(monkeypatch, vcodec="h264")
     c = as_user(make_user())
     job = c.post(
-        "/api/downloads/upload", content=b"h264 bytes", headers=upload_headers("clip.mp4")
+        "/api/files/upload", content=b"h264 bytes", headers=upload_headers("clip.mp4")
     ).json()
     link = c.post(f"/api/downloads/{job['id']}/links", json={"allow_download": False}).json()
     assert c.get(f"/api/public/watch/{link['token']}").status_code == 200
@@ -570,7 +570,7 @@ def test_a_link_to_a_suspended_owners_waiting_rebuild_says_it_is_gone(
     owner = make_user()
     c = as_user(owner)
     job = c.post(
-        "/api/downloads/upload", content=b"h264 bytes", headers=upload_headers("clip.mp4")
+        "/api/files/upload", content=b"h264 bytes", headers=upload_headers("clip.mp4")
     ).json()
     link = c.post(f"/api/downloads/{job['id']}/links", json={"allow_download": False}).json()
     db.get(MediaAsset, job["asset"]["id"]).status = "pending"