feat(nip17): support gift-wrapped private direct messages

Generalize the AUTH-gated, recipient-only delivery rule from NIP-04 to
also cover NIP-17 kind 1059 gift wraps. When the relay is configured to
require AUTH for kind 1059, only the AUTH'd recipient named in the
event's `p` tag receives it; otherwise gift wraps broadcast like any
regular event.

- relay/event.py: add `is_seal`, `is_gift_wrap`, `is_private_message`
  helpers (kinds 13, 1059)
- relay/client_connection.py: rename `_is_direct_message_for_other` ->
  `_is_private_event_for_other`; key off `is_private_message` so the
  same gating applies to kinds 4 and 1059
- relay/relay.py: advertise NIPs 17, 44, 59 in NIP-11 supported_nips
- README: document NIP-17/44/59 transport-level support
- tests/test_nip17.py: unit tests for kind classification, AUTH-gated
  1059 delivery (recipient vs non-recipient vs unauthenticated), and
  regression coverage for kind 4 gating

NIP-44 (encryption) and NIP-59 (wrap/seal) are client-side concerns;
the relay treats payloads as opaque ciphertext and stores kind 1059
like any regular event.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

relay/relay.py

@@ -116,7 +116,7 @@ class NostrRelay(BaseModel):
     ) -> dict:
         return {
             "contact": "https://t.me/lnbits",
-            "supported_nips": [1, 2, 4, 9, 11, 15, 16, 20, 22, 28, 42],
+            "supported_nips": [1, 2, 4, 9, 11, 15, 16, 17, 20, 22, 28, 42, 44, 59],
             "software": "LNbits",
             "version": "",
         }