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>

services.py

@@ -56,6 +56,7 @@ from .models import (
     Stall,
 )
 from .nostr.event import NostrEvent
+from .nostr.nip59 import unwrap_message, wrap_message
 
 
 async def create_new_order(
@@ -270,19 +271,34 @@ async def send_dm(
     other_pubkey: str,
     type_: int,
     dm_content: str,
-):
-    dm_event = merchant.build_dm_event(dm_content, other_pubkey)
+) -> DirectMessage:
+    # Wrap message to recipient via NIP-59 gift wrap
+    gift_wrap = wrap_message(
+        dm_content,
+        merchant.private_key,
+        merchant.public_key,
+        other_pubkey,
+    )
 
     dm = PartialDirectMessage(
-        event_id=dm_event.id,
-        event_created_at=dm_event.created_at,
+        event_id=gift_wrap.id,
+        event_created_at=gift_wrap.created_at,
         message=dm_content,
         public_key=other_pubkey,
         type=type_,
     )
     dm_reply = await create_direct_message(merchant.id, dm)
 
-    await nostr_client.publish_nostr_event(dm_event)
+    await nostr_client.publish_nostr_event(gift_wrap)
+
+    # Also wrap a copy to self for archival
+    self_wrap = wrap_message(
+        dm_content,
+        merchant.private_key,
+        merchant.public_key,
+        merchant.public_key,
+    )
+    await nostr_client.publish_nostr_event(self_wrap)
 
     await websocket_updater(
         merchant.id,
@@ -295,6 +311,8 @@ async def send_dm(
         ),
     )
 
+    return dm_reply
+
 
 async def compute_products_new_quantity(
     merchant_id: str, product_ids: List[str], items: List[OrderItem]
@@ -332,11 +350,15 @@ async def process_nostr_message(msg: str):
                 return
             _, event = rest
             event = NostrEvent(**event)
-            
+
+            # Deduplicate events (overlap resubscriptions may deliver duplicates)
+            if nostr_client.is_duplicate_event(event.id):
+                return
+
             if event.kind == 0:
                 await _handle_customer_profile_update(event)
-            elif event.kind == 4:
-                await _handle_nip04_message(event)
+            elif event.kind == 1059:
+                await _handle_gift_wrap(event)
             elif event.kind == 30017:
                 await _handle_stall(event)
             elif event.kind == 30018:
@@ -430,30 +452,41 @@ async def extract_customer_order_from_dm(
     return order
 
 
-async def _handle_nip04_message(event: NostrEvent):
-    
+async def _handle_gift_wrap(event: NostrEvent):
+    """Handle an incoming kind 1059 gift wrap event (NIP-59/NIP-17)."""
+
     p_tags = event.tag_values("p")
-    
-    # PRIORITY 1: Check if any recipient (p_tag) is a merchant → incoming message TO merchant
-    for p_tag in p_tags:
-        if p_tag:
-            potential_merchant = await get_merchant_by_pubkey(p_tag)
-            if potential_merchant:
-                clear_text_msg = potential_merchant.decrypt_message(event.content, event.pubkey)
-                await _handle_incoming_dms(event, potential_merchant, clear_text_msg)
-                return  # IMPORTANT: Return immediately to prevent double processing
-    
-    # PRIORITY 2: If no recipient merchant found, check if sender is a merchant → outgoing message FROM merchant
-    sender_merchant = await get_merchant_by_pubkey(event.pubkey)
-    if sender_merchant:
-        assert len(event.tag_values("p")) != 0, "Outgoing message has no 'p' tag"
-        clear_text_msg = sender_merchant.decrypt_message(
-            event.content, event.tag_values("p")[0]
+    if not p_tags:
+        logger.warning(f"[NOSTRMARKET] ⚠️ Gift wrap has no p-tag: {event.id}")
+        return
+
+    # The p-tag identifies the recipient of the gift wrap
+    recipient_pubkey = p_tags[0]
+    merchant = await get_merchant_by_pubkey(recipient_pubkey)
+    if not merchant:
+        logger.warning(
+            f"[NOSTRMARKET] ⚠️ No merchant found for gift wrap recipient: {recipient_pubkey}"
         )
-        await _handle_outgoing_dms(event, sender_merchant, clear_text_msg)
-        return  # IMPORTANT: Return immediately
-    
-    # No merchant found in either direction
+        return
+
+    try:
+        rumor = unwrap_message(event, merchant.private_key)
+    except Exception as ex:
+        logger.error(f"[NOSTRMARKET] ❌ Failed to unwrap gift wrap {event.id}: {ex}")
+        return
+
+    sender_pubkey = rumor.pubkey
+
+    if sender_pubkey == merchant.public_key:
+        # This is a self-addressed wrap (outgoing message archive)
+        # Extract the actual recipient from the rumor's p-tags
+        rumor_p_tags = rumor.tag_values("p")
+        if rumor_p_tags:
+            await _handle_outgoing_dms(rumor, merchant, rumor.content)
+        return
+
+    # Incoming message from a customer
+    await _handle_incoming_dms(rumor, merchant, rumor.content)
 
 
 async def _handle_incoming_dms(
@@ -553,16 +586,21 @@ async def _persist_dm(
 async def reply_to_structured_dm(
     merchant: Merchant, customer_pubkey: str, dm_type: int, dm_reply: str
 ):
-    dm_event = merchant.build_dm_event(dm_reply, customer_pubkey)
+    gift_wrap = wrap_message(
+        dm_reply,
+        merchant.private_key,
+        merchant.public_key,
+        customer_pubkey,
+    )
     dm = PartialDirectMessage(
-        event_id=dm_event.id,
-        event_created_at=dm_event.created_at,
+        event_id=gift_wrap.id,
+        event_created_at=gift_wrap.created_at,
         message=dm_reply,
         public_key=customer_pubkey,
         type=dm_type,
     )
     await create_direct_message(merchant.id, dm)
-    await nostr_client.publish_nostr_event(dm_event)
+    await nostr_client.publish_nostr_event(gift_wrap)
 
     await websocket_updater(
         merchant.id,