feat(files): every file sits in a library folder, and keeps its own expiry

The Files library starts here. A user's every download, clip and upload now
lands in a source folder made per origin (YouTube, Plex clips, uploads, one per
other site), decided once in downloads/folders.py at every place a job is made.
How long a file stays moves from the shared asset to the user's job: stamped
from its folder when it is done, never for a file the user keeps. The GC warns
each user once, then takes the expired file out of their library; the cache cap
never evicts a kept file, and an evicted one leaves every holder's library.

The media mount is /files now (FILES_ROOT, FILES_HOST_PATH, volume
siftlode_files); docs/self-hosting.md says how to carry an old volume over.
Migration 0102 places every existing job and carries each done file's expiry
over from its asset.

safe_abs_path takes a missing path and answers None, which drops one baselined
type error.

backend/app/routes/downloads.py

@@ -145,6 +145,10 @@ def _serialize(job: DownloadJob, asset: MediaAsset | None) -> dict:
         "edit_spec": job.edit_spec,
         "can_download": ready and job.status == JobStatus.DONE,
         "expired": job.asset_id is None and job.status == JobStatus.DONE,
+        # The Files library: its folder, whether the user keeps it, and when it goes (NULL = never).
+        "folder_id": job.folder_id,
+        "kept": job.kept,
+        "expires_at": iso(job.expires_at),
         "asset": None
         if asset is None
         else {
@@ -163,7 +167,6 @@ def _serialize(job: DownloadJob, asset: MediaAsset | None) -> dict:
             "media_kind": asset.media_kind,
             "uploader_url": asset.uploader_url,
             "thumbnail_url": asset.thumbnail_url,
-            "expires_at": iso(asset.expires_at),
         },
     }
 
@@ -495,7 +498,7 @@ async def spool_upload(
     # already an int or a header in hand: touching an ORM attribute below would reload `user` and
     # check a connection straight back out.
     release_read_connection(db)
-    staging = Path(settings.download_root) / ".staging" / "uploads"
+    staging = Path(settings.files_root) / ".staging" / "uploads"
     staging.mkdir(parents=True, exist_ok=True)
     fd, tmp = tempfile.mkstemp(dir=str(staging))
     tmp_path = Path(tmp)
@@ -871,7 +874,7 @@ def download_file(
     asset = db.get(MediaAsset, job.asset_id) if job.asset_id else None
     if asset is None or asset.status != "ready" or not asset.rel_path:
         raise refused(409, "download_not_ready")
-    path = storage.safe_abs_path(settings.download_root, asset.rel_path)
+    path = storage.safe_abs_path(settings.files_root, asset.rel_path)
     if path is None:
         raise not_found("file")
 
@@ -946,7 +949,7 @@ def storyboard_meta(
     asset = db.get(MediaAsset, job.asset_id) if job.asset_id else None
     if asset is None or asset.status != "ready" or not asset.rel_path:
         raise refused(409, "download_not_ready")
-    meta = editmod.ensure_storyboard(asset, settings.download_root)
+    meta = editmod.ensure_storyboard(asset, settings.files_root)
     if meta is None:
         return {"available": False}
     db.commit()  # persist storyboard_path if it was just generated
@@ -967,7 +970,7 @@ def poster_image(job_id: int, user: User = Depends(current_user), db: Session =
     asset = db.get(MediaAsset, job.asset_id) if job.asset_id else None
     if asset is None or not asset.poster_path:
         raise not_found("poster")
-    path = storage.safe_abs_path(settings.download_root, asset.poster_path)
+    path = storage.safe_abs_path(settings.files_root, asset.poster_path)
     if path is None:
         raise not_found("poster")
     return FileResponse(path, media_type="image/jpeg")
@@ -981,7 +984,7 @@ def storyboard_image(
     asset = db.get(MediaAsset, job.asset_id) if job.asset_id else None
     if asset is None or not asset.storyboard_path:
         raise not_found("filmstrip")
-    path = storage.safe_abs_path(settings.download_root, asset.storyboard_path)
+    path = storage.safe_abs_path(settings.files_root, asset.storyboard_path)
     if path is None:
         raise not_found("filmstrip")
     return FileResponse(path, media_type="image/jpeg")