feat: CSV export and employee payslip view

Two audiences the super-user API did not serve.

Accounting gets `GET /api/v1/payouts.csv`, filterable by contract, status
and date range. The range bounds the *payday* rather than the row
timestamp, so a period contains the paydays that belong to it even when one
of them took three days of retries to settle — otherwise a late retry lands
in the wrong month's export.

Every exported field is neutralised against spreadsheet formula injection.
`detail` carries exception text and the memo carries operator input, and a
cell beginning `=`, `+`, `-` or `@` executes when the file is opened. Worth
the eight lines: this file is written specifically to be opened in somebody
else's spreadsheet.

Employees get `/api/v1/my/payouts`, `/my/payouts.csv` and `/my/contracts`
on a **separate router** gated by a wallet invoice key rather than by
super-user rights. Separate router so it cannot inherit — or accidentally
shed — the wrong gate. Scoped to the wallet rather than the account because
an invoice key names exactly one wallet, leaving no lookup that could widen
the result to a sibling wallet the key does not cover.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018jy52j9GRZ6XKa1Zt21LLj

README.md

@@ -25,6 +25,13 @@ workspace `CLAUDE.md`.)
 Add `payroll` to `LNBITS_ADMIN_EXTENSIONS` so the extension is hidden from
 ordinary users in the UI as well.
 
+The exception is the employee-facing surface — `/api/v1/my/payouts`,
+`/my/payouts.csv`, `/my/contracts` — which is gated on a wallet **invoice
+key** instead. Whoever holds the read key for a wallet can see what payroll
+paid into that wallet, and nothing else. It stays reachable regardless of
+`LNBITS_ADMIN_EXTENSIONS`, which governs UI visibility rather than route
+mounting.
+
 ## Development
 
 ```