feat(files): one uploads/ directory, for files and chat alike

Correction to the previous commit. Uploads went wherever the member happened to
be looking, which spreads chat attachments through the tree and makes the
destination a client-supplied path — surface that had to be defended. Everything
a member sends now lands in `uploads/` at the root of the shared directory:
visible, one place, easy for the operator to look into or empty.

Chat attachments go there too, so the separate out-of-tree thumbs directory is
not needed and is not built. They were already ordinary uploads; now they are
ordinary uploads that land somewhere sensible.

The destination is chosen by the node, so a client naming somewhere else changes
nothing — the traversal surface simply is not there on this path. safe_subdir()
remains for dir_create, where the path genuinely does come from the client, and
keeps its tests.

One shared directory means name collisions are ordinary rather than adversarial:
every camera produces IMG_1234.jpg. The node finds a free name — "IMG_1234 (2).jpg"
— and reports it in the ack, because a chat message has to point at the file that
was actually written and not at someone else's. Nothing is ever replaced, which
is the property the per-user quarantine existed for (C5a) and the one the tests
assert; they fail if the free-name search is removed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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