Replace NIP-04 messaging with NIP-17 (NIP-44 + NIP-59 gift wrapping)

Modernize the entire customer-merchant communication layer from deprecated
NIP-04 (kind 4, AES-256-CBC) to NIP-17 private direct messages using
NIP-44 v2 encryption (ChaCha20 + HMAC-SHA256) and NIP-59 gift wrapping
(rumor/seal/gift-wrap protocol). No backwards compatibility retained.

New modules:
- nostr/nip44.py: NIP-44 v2 encryption verified against official spec vectors
- nostr/nip59.py: NIP-59 gift wrap with wrap/unwrap convenience functions
- tests/: 44 unit tests for NIP-44 and NIP-59

Key changes:
- Subscription filters: kind 4 → kind 1059 gift wraps
- Message handler: _handle_nip04_message → _handle_gift_wrap (unwrap + route)
- send_dm/reply_to_structured_dm: NIP-59 gift wrap to recipient + self-archive
- Merchant model: removed NIP-04 crypto methods (decrypt/encrypt/build_dm_event)
- helpers.py: removed NIP-04 functions, kept Schnorr signing + key normalization
- views_api.py: consolidated DM sending through send_dm() service function

Reliability improvements:
- Event deduplication via bounded LRU set in NostrClient
- Subscription health monitor (resubscribes after 120s of silence)
- Preserved 5-minute lenient time window from prior work

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

models.py

@@ -7,12 +7,7 @@ from typing import Any, List, Optional, Tuple
 from lnbits.utils.exchange_rates import btc_price, fiat_amount_as_satoshis
 from pydantic import BaseModel
 
-from .helpers import (
-    decrypt_message,
-    encrypt_message,
-    get_shared_secret,
-    sign_message_hash,
-)
+from .helpers import sign_message_hash
 from .nostr.event import NostrEvent
 
 ######################################## NOSTR ########################################
@@ -67,28 +62,6 @@ class Merchant(PartialMerchant, Nostrable):
     def sign_hash(self, hash_: bytes) -> str:
         return sign_message_hash(self.private_key, hash_)
 
-    def decrypt_message(self, encrypted_message: str, public_key: str) -> str:
-        encryption_key = get_shared_secret(self.private_key, public_key)
-        return decrypt_message(encrypted_message, encryption_key)
-
-    def encrypt_message(self, clear_text_message: str, public_key: str) -> str:
-        encryption_key = get_shared_secret(self.private_key, public_key)
-        return encrypt_message(clear_text_message, encryption_key)
-
-    def build_dm_event(self, message: str, to_pubkey: str) -> NostrEvent:
-        content = self.encrypt_message(message, to_pubkey)
-        event = NostrEvent(
-            pubkey=self.public_key,
-            created_at=round(time.time()),
-            kind=4,
-            tags=[["p", to_pubkey]],
-            content=content,
-        )
-        event.id = event.event_id
-        event.sig = self.sign_hash(bytes.fromhex(event.id))
-
-        return event
-
     @classmethod
     def from_row(cls, row: dict) -> "Merchant":
         merchant = cls(**row)