NIP-52E/52R: private calendar events, busy lists and recurring event support

[geralt-debugs]

Introduces two companion extensions to NIP-52:

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NIP-52R - Recurring Events

Extends NIP-52 with RRULE-based recurrence using L/l label tags inside event payloads.

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NIP-52E - Private Calendar Events

Adds a privacy layer on top of NIP-52 using a view key pattern (NIP-44 self-encryption):

• 32678 - Private time-based calendar event (encrypted equivalent of 31923)
• 32681 - Private day event (encrypted equivalent of 31922)
• 1052 - Calendar event gift wrap (NIP-59) for distributing view keys to participants
• 32123 - Private calendar list (self-encrypted, owner-only)
• 31926 - Public busy list (signals unavailability without leaking event details)

Only the d tag is public; all event content lives in the encrypted content field. Participants receive the
viewKey (nsec) via gift wrap and can decrypt independently of the author.

Full protocol flows are specified for: creating a private event, receiving and accepting an invitation, and
loading calendar events at startup.

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Closing #2027 in favour of this

[geralt-debugs]

@fiatjaf @abh3po @staab @vitorpamplona