Add calendar/address-book sharing backend

Introduce internal/db, a small SQLite-backed store (pure-Go
modernc.org/sqlite, no CGO) at <data_dir>/nidus.db holding
calendar_shares and addressbook_shares grant tables (owner, resource
name, shared-with user, read/write permission). This is the first step
towards user management and a web UI: a real datastore that a future
admin CLI/UI can build on, instead of the static config.yaml.

Wire sharing into the CalDAV/CardDAV backends:
- ListCalendars/ListAddressBooks now also include resources shared with
  the requesting user, exposed under the synthetic local name
  "<owner>~<name>" in the grantee's own home-set — no separate account,
  no data copying, the object still physically lives under the owner's
  store.Store namespace.
- All read paths (Get/List/QueryCalendarObjects, address book
  equivalents) resolve the synthetic name back to (owner, real name) and
  require any share (read or write) to exist.
- All write paths (Put/Delete object, DeleteCalendar/AddressBook)
  additionally require a write-permission share; read-only shares get a
  403 Forbidden.
- CreateCalendar/CreateAddressBook remain scoped to the acting user's own
  namespace — sharing an existing collection is done via ShareCalendar/
  ShareAddressBook, not by creating one directly in someone else's name.

Add internal/db/shares_test.go (grant/lookup/update/unshare/list
semantics) and internal/{caldav,carddav}/backend_test.go (shared
calendar/address book visibility, write permission enforcement,
unauthorized access rejection). Update README (features, new "Sharing
calendars and address books" section, project layout, dependencies) and
copilot-instructions.md to document the new package and sharing model.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

go.sum

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