feat: turn pages into simple databases

[AmrMsCLL]

Stacked on #45.

What changes

• Any page can be turned into a database. Pages inside it get typed properties: text, number, select, multi-select, date, checkbox, person (a member of the space) and link (http/https).
• Four views, with the chosen one saved on the database: a table with inline editing, a board grouped by a select property (drag cards between columns, add in a column), a list, and a month calendar by a date property (add a page on a day).
• A "Properties" dialog adds, renames, retypes and removes properties and their options.
• A page inside a database shows its properties above its content.
• Changing properties drops values that no longer fit: removed properties, changed types and removed options. Turning a database back into a page, which asks for confirmation first, deletes its pages' property values but keeps the pages.
• The month-grid range used by Agenda moved to agenda.ts (monthDays) so the calendar view shares it.

API

• GET/PUT/DELETE /notes/:id/database
• PATCH /notes/:id/properties with { values: { [propertyId]: value | null } }
• Notes gain database: boolean. GET /notes/:id gains row (the parent database's properties and this page's values, or null).

Migration

20261005080000_note_databases creates database_schemas (item_id pk, properties jsonb, view, group_by, date_by, updated_at) and note_properties (item_id, property_id, value jsonb), both cascading from items.

Verification

• Server integration: test/integration/note-databases.spec.ts, test/integration/notes.spec.ts (8 passed).
• Frontend: test/notes.spec.tsx, test/agenda.spec.tsx (12 passed).
• Lint and build pass. Checked on the local stack: turning a page into a database, setting values on its pages, and the board and calendar views.