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>

nostr/nostr_client.py

@@ -1,6 +1,8 @@
 import asyncio
 import json
+import time
 from asyncio import Queue
+from collections import OrderedDict
 from threading import Thread
 from typing import Callable, List, Optional
 
@@ -12,6 +14,8 @@ from lnbits.helpers import encrypt_internal_message, urlsafe_short_hash
 
 from .event import NostrEvent
 
+MAX_SEEN_EVENTS = 1000
+
 
 class NostrClient:
     def __init__(self):
@@ -20,6 +24,8 @@ class NostrClient:
         self.ws: Optional[WebSocketApp] = None
         self.subscription_id = "nostrmarket-" + urlsafe_short_hash()[:32]
         self.running = False
+        self._seen_events: OrderedDict[str, None] = OrderedDict()
+        self.last_event_at: float = 0
 
     @property
     def is_websocket_connected(self):
@@ -64,11 +70,21 @@ class NostrClient:
                 logger.warning(ex)
                 await asyncio.sleep(60)
 
+    def is_duplicate_event(self, event_id: str) -> bool:
+        """Check if an event has been seen recently. Returns True if duplicate."""
+        if event_id in self._seen_events:
+            return True
+        self._seen_events[event_id] = None
+        if len(self._seen_events) > MAX_SEEN_EVENTS:
+            self._seen_events.popitem(last=False)
+        return False
+
     async def get_event(self):
         value = await self.recieve_event_queue.get()
         if isinstance(value, ValueError):
             logger.error(f"[NOSTRMARKET] ❌ Queue returned error: {value}")
             raise value
+        self.last_event_at = time.time()
         return value
 
     async def publish_nostr_event(self, e: NostrEvent):
@@ -134,13 +150,13 @@ class NostrClient:
             logger.debug(ex)
 
     def _filters_for_direct_messages(self, public_keys: List[str], since: int) -> List:
-        in_messages_filter = {"kinds": [4], "#p": public_keys}
-        out_messages_filter = {"kinds": [4], "authors": public_keys}
+        # NIP-17/NIP-59: subscribe to kind 1059 gift wraps addressed to our merchants.
+        # With gift wrapping, outgoing messages are self-wrapped (same p-tag filter).
+        gift_wrap_filter: dict = {"kinds": [1059], "#p": public_keys}
         if since and since != 0:
-            in_messages_filter["since"] = since
-            out_messages_filter["since"] = since
+            gift_wrap_filter["since"] = since
 
-        return [in_messages_filter, out_messages_filter]
+        return [gift_wrap_filter]
 
     def _filters_for_stall_events(self, public_keys: List[str], since: int) -> List:
         stall_filter = {"kinds": [30017], "authors": public_keys}