* fix: drop extra='allow' on FolderForm and FolderUpdateForm
These request models were configured to accept arbitrary extra fields,
which were then merged into the folder row via form_data.model_dump().
In insert_new_folder the server-assigned user_id is placed before the
form spread, so a client-supplied user_id in the request body would
override it and the folder would be persisted against another account.
Strictly typed inputs are the correct shape for these endpoints — the
client has no legitimate reason to send fields beyond the declared
ones, and dropping extra='allow' closes the mass-assignment sink at
the validation layer instead of relying on every callsite to merge
fields in the right order.
* fix: reject unknown fields on FolderForm and FolderUpdateForm
Address review feedback: dropping extra='allow' fell back to Pydantic
v2's default extra='ignore', which only silently drops unknown fields
instead of rejecting them. The intent for these request models is a
strict input contract — fail fast when a client sends anything the
server does not expect — so explicitly set extra='forbid'. This also
makes the hardening visible in the form definition rather than implicit
in the default.
