feat: split shared subscriptions and settle up

[AmrMsCLL]

Stacked on #50. This is 11h of as2026-10-05-strata-phase-11-next-batch-2.md.

What changes

• In a shared space, editors can Split a subscription: who pays, and which members share it, with weights from 1 to 100. Percentages are shown as you edit.
• The space's Recurring tab gains a Split costs card for each month. It lists the renewals that actually fall in that month, using the same recurrence code as reminders and stopping at a cancellation. It nets what each pair of members owes per currency.
• Mark as paid records a payment for the month. Only the person who was paid can record it, so a debtor can't clear their own debt. Either of the two people can undo it, and other members only see it.
• The split model is keyed by item (cost_splits holds the payer, cost_shares the weights), so events can reuse it later.
• Member (userId, displayName) moved to spaces.tsx. The database property fields from feat: turn pages into simple databases #46 now import it from there too.

Migration

20261005160000_cost_splitting creates cost_splits (item_id pk, payer_id), cost_shares (item_id, user_id, weight) and settlements (id, space_id, from_user_id, to_user_id, amount_minor, currency, month, created_at). All of them cascade from items, users and spaces.

API

• PUT/DELETE /subscriptions/:id/split
• GET /spaces/:id/balances?month=YYYY-MM
• POST /spaces/:id/settlements, DELETE /settlements/:id
• Subscriptions gain split ({ payerId, shares } or null).

Verification

• Server integration: test/integration/cost-splits.spec.ts and test/integration/subscriptions.spec.ts (5 passed).
• Frontend: test/subscriptions.spec.tsx (6 passed) and test/notes.spec.tsx (10 passed, for the moved Member type).
• Lint and build pass.