CSV export silently truncates at 1000 cases with no pagination

[royalpinto007]

app/api/export/route.ts is the machine-readable view of the registry, and it silently truncates:

.eq("status", "approved")
.order("vote_score", { ascending: false })
.limit(1000);

Once the registry passes 1000 approved cases, /api/export returns the 1000 highest-voted ones and says nothing about the rest. A consumer building a dataset has no way to detect that they got a partial answer, and because the ordering is by vote_score the truncation is also biased toward popular cases rather than being a clean prefix.

There are related rough edges in the same handler worth doing at the same time:

• No pagination parameters at all, so there is no way to ask for the remainder.
• No Content-Disposition header, so the CSV renders in the browser rather than downloading.
• No documented schema. The headers array in this file is the only description of the columns that exists anywhere.

Suggested shape:

• Accept ?limit= and ?offset= (or a cursor on case_number) with a sane maximum, validated with Zod the way the other routes validate input.
• Return a header or a trailing metadata row indicating the total count so truncation is detectable.
• Add Content-Disposition: attachment; filename="agentpostmortem-export.csv".
• Document the columns in the README next to the rest of the registry description.

Note that #20 covers CSV formula injection in the same file. Keep the two changes separate so they review independently.

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