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/test_body_limit.py

@@ -221,7 +221,7 @@ def test_the_default_ceiling_clears_the_largest_body_a_default_route_takes():
 CAPPED_BODY_SITES = {
     ("diagnostics.py", "_parsed_events", "capped_body", "BODY_MAX_BYTES"),
     ("debug_sessions.py", "_image_body", "capped_body", "debugsession.SCREENSHOT_MAX_BYTES"),
-    ("downloads.py", "spool_upload", "capped_stream_to", "cap"),
+    ("files.py", "spool_upload", "capped_stream_to", "cap"),
 }
 
 
@@ -467,7 +467,7 @@ def test_every_body_reading_site_hands_its_pooled_connection_back_first():
     assert not missing, (
         "a route reads its own request body while still inside its read transaction, pinning a "
         f"pooled connection for the whole transfer: {missing}. Call "
-        "db.release_read_connection(db) before the await — see routes/downloads.py:spool_upload."
+        "db.release_read_connection(db) before the await — see routes/files.py:spool_upload."
     )
 
 
@@ -540,7 +540,7 @@ def test_no_route_with_a_raised_ceiling_declares_a_body_parameter(method, templa
         "its dependencies, so FastAPI reads it before the first dependency runs and the raised "
         "ceiling is all that stands between an anonymous caller and the whole allowance. Read the "
         "body yourself, from a dependency declared AFTER the auth one — see "
-        "routes/downloads.py:spool_upload."
+        "routes/files.py:spool_upload."
     )