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>

tasks.py

@@ -1,4 +1,5 @@
 import asyncio
+import time
 from asyncio import Queue
 
 from lnbits.core.models import Payment
@@ -9,9 +10,13 @@ from .nostr.nostr_client import NostrClient
 from .services import (
     handle_order_paid,
     process_nostr_message,
+    resubscribe_to_all_merchants,
     subscribe_to_all_merchants,
 )
 
+HEALTH_CHECK_INTERVAL = 30  # seconds between health checks
+STALE_THRESHOLD = 120  # seconds without events before resubscribing
+
 
 async def wait_for_paid_invoices():
     invoice_queue = Queue()
@@ -35,17 +40,38 @@ async def on_invoice_paid(payment: Payment) -> None:
 
 
 async def wait_for_nostr_events(nostr_client: NostrClient):
-    logger.info("[NOSTRMARKET DEBUG] Starting wait_for_nostr_events task")
+    logger.info("[NOSTRMARKET] Starting wait_for_nostr_events task")
     while True:
         try:
-            logger.info("[NOSTRMARKET DEBUG] Subscribing to all merchants...")
+            logger.info("[NOSTRMARKET] Subscribing to all merchants...")
             await subscribe_to_all_merchants()
 
             while True:
-                logger.debug("[NOSTRMARKET DEBUG] Waiting for nostr event...")
                 message = await nostr_client.get_event()
-                logger.info(f"[NOSTRMARKET DEBUG] Received event from nostr_client: {message[:100]}...")
                 await process_nostr_message(message)
         except Exception as e:
-            logger.warning(f"[NOSTRMARKET DEBUG] Subscription failed. Will retry in 10 seconds: {e}")
+            logger.warning(f"[NOSTRMARKET] Subscription failed. Retrying in 10s: {e}")
             await asyncio.sleep(10)
+
+
+async def subscription_health_monitor(nostr_client: NostrClient):
+    """
+    Periodically check if events are flowing. If no events have been
+    received for STALE_THRESHOLD seconds, force a resubscription with
+    overlap to catch any missed events.
+    """
+    logger.info("[NOSTRMARKET] Starting subscription health monitor")
+    while True:
+        await asyncio.sleep(HEALTH_CHECK_INTERVAL)
+        try:
+            if not nostr_client.is_websocket_connected:
+                continue
+
+            elapsed = time.time() - nostr_client.last_event_at
+            if nostr_client.last_event_at > 0 and elapsed > STALE_THRESHOLD:
+                logger.warning(
+                    f"[NOSTRMARKET] ⚠️ No events for {elapsed:.0f}s, resubscribing..."
+                )
+                await resubscribe_to_all_merchants()
+        except Exception as e:
+            logger.error(f"[NOSTRMARKET] Health monitor error: {e}")