feat(files): the Files library API — folders, moving and keeping files, upload into a folder, the file's hash

/api/files is the library's home: list a user's done files (by folder, searched, sorted, or the
ones shared with them), rename / move / keep / delete a file, make, rename and delete own folders
(a deleted folder's files go back to their source folders), and set how long any folder keeps a
file — done files shift by the difference, never closer than the GC's warning window.

An upload can name its folder (an own folder or Uploads), checked before the body is read and again
before the file lands; it is hashed while it streams, and any file can be hashed on first ask. The
download says the hash (Repr-Digest) once it is known; a new file at an asset's path forgets it.

Uploads, deletes, keeps, moves and folder changes are audited (ids only). The fileless "expired"
rows of the old retention are deleted by migration 0103: an expired file now leaves the library.

The shared route helpers moved to routes/files.py; the old /api/downloads library routes delegate
to the new ones until the client moves over.

backend/tests/conftest.py

@@ -254,20 +254,24 @@ def as_user(db, override_dep, _configured):
 
 @pytest.fixture
 def upload_headers():
-    """Build the headers `POST /api/downloads/upload` takes (the file is the raw body).
+    """Build the headers the upload routes take (the file is the raw body).
 
     Shared, because two test modules drive that route and a second copy of this is exactly the drift
     that lets the wire shape change in one place only. The body IS the file now; the names ride as
     percent-encoded headers (`quote` mirrors the client's `encodeURIComponent`, since headers are
     latin-1 and a Hungarian filename would not survive raw)."""
 
-    def _headers(filename: str, display_name: str | None = None) -> dict[str, str]:
+    def _headers(
+        filename: str, display_name: str | None = None, folder: int | str | None = None
+    ) -> dict[str, str]:
         headers = {
             "content-type": "application/octet-stream",
             "x-upload-filename": quote(filename),
         }
         if display_name is not None:
             headers["x-upload-display-name"] = quote(display_name)
+        if folder is not None:
+            headers["x-upload-folder"] = str(folder)
         return headers
 
     return _headers