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/downloads/edit.py

@@ -732,7 +732,7 @@ def thumbnail_to_jpg(src: Path | None, dest: Path, timeout_s: int = 30) -> Path
     return dest if dest.exists() else None
 
 
-def ensure_poster(asset, download_root: str, timeout_s: int = 30) -> str | None:
+def ensure_poster(asset, files_root: str, timeout_s: int = 30) -> str | None:
     """Generate a poster frame for a ready asset that has no thumbnail (e.g. a direct media URL
     like a reddit HLS manifest, which yt-dlp can't get a thumbnail for). Written as the media's
     `<base>.jpg` sidecar — so Plex uses it too and `delete_asset_files` cleans it up — and returned
@@ -740,7 +740,7 @@ def ensure_poster(asset, download_root: str, timeout_s: int = 30) -> str | None:
     short clip) dodges black intro frames."""
     if not asset.rel_path:
         return None
-    root = Path(download_root)
+    root = Path(files_root)
     src = root / asset.rel_path
     if not src.exists():
         return None
@@ -756,7 +756,7 @@ def ensure_poster(asset, download_root: str, timeout_s: int = 30) -> str | None:
     return rel if dest.exists() else None
 
 
-def ensure_storyboard(asset, download_root: str, timeout_s: int = 90) -> dict | None:
+def ensure_storyboard(asset, files_root: str, timeout_s: int = 90) -> dict | None:
     """Return the filmstrip meta for a ready source asset, generating the sprite on first use.
 
     Best-effort + time-bounded: for a very long/high-res source the one-pass decode can be slow,
@@ -768,7 +768,7 @@ def ensure_storyboard(asset, download_root: str, timeout_s: int = 90) -> dict |
     if not asset.rel_path:
         return None
     geom = storyboard_geometry(asset.duration_s)
-    root = Path(download_root)
+    root = Path(files_root)
     rel = f".storyboards/asset-{asset.id}.jpg"
     dest = root / rel