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/app/routes/_common.py

@@ -6,7 +6,7 @@ and "fetch a row this user owns or 404" idioms have one definition, one status c
 one existence-leak policy.
 """
 
-from typing import IO, Any, TypeVar
+from typing import Any, Protocol, TypeVar
 
 from fastapi import HTTPException, Request
 from pydantic import BaseModel, ConfigDict, Field
@@ -122,8 +122,14 @@ async def capped_body(request: Request, limit: int) -> bytes:
     return bytes(body)
 
 
+class ByteSink(Protocol):
+    """Where `capped_stream_to` writes: a file, or anything that takes the bytes on their way to one."""
+
+    def write(self, data: bytes, /) -> object: ...
+
+
 async def capped_stream_to(
-    request: Request, out: IO[bytes], limit: int, refusal: HTTPException
+    request: Request, out: ByteSink, limit: int, refusal: HTTPException
 ) -> int:
     """`capped_body`'s sibling for a body too large to hold: the same counting rule, written
     straight to `out` instead of accumulated, returning the size.