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_library_progress.py

@@ -9,13 +9,26 @@ cannot reach, is ever read or written. DB-backed (real Postgres).
 import pytest
 from sqlalchemy import select
 
-from app.models import DownloadJob, DownloadShare, LibraryState
+from app.models import DownloadJob, DownloadShare, LibraryState, MediaAsset
 from app.routes._common import ProgressIn, kept_position
 
 
 def _done_job(db, user) -> DownloadJob:
+    asset = MediaAsset(
+        source_kind="url",
+        source_ref="https://example.com/a.mp4",
+        format_sig=f"sig-{user.id}",
+        status="ready",
+        rel_path=f"a-{user.id}.mp4",
+        ref_count=1,
+        media_kind="video",
+        container="mp4",
+    )
+    db.add(asset)
+    db.flush()
     job = DownloadJob(
         user_id=user.id,
+        asset_id=asset.id,
         source_kind="url",
         source_ref="https://example.com/a.mp4",
         status="done",
@@ -59,7 +72,7 @@ def test_a_checkpoint_is_listed_back_on_the_item(db, make_user, as_user):
         "position_seconds": 754,
     }
 
-    [item] = c.get("/api/downloads").json()
+    [item] = c.get("/api/files").json()
     assert item["position_seconds"] == 754
     # A second checkpoint moves it rather than adding a row.
     assert _checkpoint(c, job.id, 900) == 900
@@ -69,7 +82,7 @@ def test_a_checkpoint_is_listed_back_on_the_item(db, make_user, as_user):
 def test_an_item_never_played_lists_zero(db, make_user, as_user):
     me = make_user()
     _done_job(db, me)
-    [item] = as_user(me).get("/api/downloads").json()
+    [item] = as_user(me).get("/api/files").json()
     assert item["position_seconds"] == 0
 
 
@@ -110,8 +123,11 @@ def test_a_recipient_keeps_their_own_place_in_a_shared_item(db, make_user, as_us
     assert _checkpoint(as_user(owner), job.id, 100) == 100
     assert _checkpoint(as_user(friend), job.id, 2000) == 2000
 
-    assert as_user(owner).get("/api/downloads").json()[0]["position_seconds"] == 100
-    assert as_user(friend).get("/api/downloads/shared").json()[0]["position_seconds"] == 2000
+    assert as_user(owner).get("/api/files").json()[0]["position_seconds"] == 100
+    assert (
+        as_user(friend).get("/api/files", params={"scope": "shared"}).json()[0]["position_seconds"]
+        == 2000
+    )
 
 
 def test_an_item_the_caller_cannot_play_is_a_404_and_writes_nothing(db, make_user, as_user):