Two lower-severity findings from the audit (todo.md tasks 6 and 7).
A spreadsheet executes a cell that begins with =, +, -, @, tab or CR, so a
vendor named `=cmd|' /C calc'!A0` runs when someone opens the CSV export
(CWE-1236). Such cells are now prefixed with a single quote, which spreadsheets
read as "this is literal text" and do not display. Only strings are escaped --
numbers reach the writer as numeric types, so a negative number cannot be
mistaken for a formula and must not grow a stray quote. Header names need no
escaping: they are fixed attribute names or extra-field keys, which are already
constrained to ^[a-z0-9_]+$.
Exports also gained Content-Disposition, so they download under a sensible name
rather than rendering in the browser.
Extra-field values had no size limit and the columns are Text(), so a single
request could store megabytes: unbounded database growth, and the resulting
websocket event overflowed the 1 MiB default frame limit, which killed live
updates for every connected client. Values are now capped at the same 65535
characters as settings. The number of extra fields per entity is capped too --
the per-entity key count was already bounded, since unknown keys are rejected,
but the registry itself could be grown without limit, and it is cached in memory
and embedded in every entity response.
Finally, a setting larger than the column limit returned 500; the ValueError is
now caught and returned as a 400.
Verified end-to-end: a 2 MB extra value is refused on both create and update, an
oversized setting returns 400 and leaves the setting unset, normal values still
work, and a vendor named `=cmd|' /C calc'!A0` exports as `'=cmd|' /C calc'!A0`
while the JSON export keeps the raw name.
