Update OKF docs bundle, README, and CLAUDE.md for today's features

Documents the shift planning and closure/payroll modules (new tables,
routes, stores), the dark redesign, and the docker-compose-broken-host
workaround. Adds CLAUDE.md pointing agents at docs/. Translates README to
English and keeps host-specific infrastructure details out of the repo.

docs/data-model/shift-signups.md

@@ -0,0 +1,48 @@
+---
+type: SQLite Table
+title: shift_signups
+description: An employee's claim on a shift slot — approved or cancelled.
+resource: backend/src/db/index.ts
+tags: [data-model, sqlite, shift-planning]
+timestamp: 2026-08-16T00:00:00Z
+---
+
+# shift_signups
+
+# Schema
+
+| Column | Type | Description |
+|---|---|---|
+| `id` | INTEGER PK | autoincrement |
+| `slot_id` | INTEGER | references [shift_slots](./shift-slots.md) |
+| `employee_id` | INTEGER | references [employees](./employees.md) |
+| `status` | TEXT | `approved` \| `cancelled` |
+| `created_at` | TEXT | ISO 8601 UTC |
+| `cancelled_at` / `cancelled_by` | TEXT \| NULL | when, and by whom (admin email or the employee's own) |
+
+Cancelling never deletes the row (`status = 'cancelled'`), so who was ever
+signed up for a slot stays visible in history.
+
+# Signup rules
+
+Enforced in `signupInternal` (`backend/src/services/shiftPlanning.ts`), in
+this order, identically whether the employee signs themselves up or the
+admin assigns them:
+
+1. Slot exists and isn't `closed`.
+2. Not already approved for this exact slot.
+3. **No time overlap** with any of the employee's other approved slots,
+   anywhere — not just the same day. Computed from real start/end
+   `Date`s (`slotStart`/`slotEnd`), so a slot whose `end_time <= start_time`
+   correctly counts as ending the next day.
+4. Slot isn't already at capacity.
+
+All of this runs synchronously in a single request — better-sqlite3 is
+synchronous, so there's no `LockService`-style mutex needed the way the
+source Apps Script version required
+(`signupEmployeeToSlot_` in `gscript/ShiftPlanningService.js`).
+
+# Related
+
+- [shift_slots](./shift-slots.md)
+- [Shift planning API](/docs/api/shifts-routes.md)