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>

__init__.py

@@ -27,7 +27,11 @@ def nostrmarket_renderer():
 nostr_client: NostrClient = NostrClient()
 
 
-from .tasks import wait_for_nostr_events, wait_for_paid_invoices  # noqa
+from .tasks import (  # noqa
+    subscription_health_monitor,
+    wait_for_nostr_events,
+    wait_for_paid_invoices,
+)
 from .views import *  # noqa
 from .views_api import *  # noqa
 
@@ -65,4 +69,13 @@ def nostrmarket_start():
     task3 = create_permanent_unique_task(
         "ext_nostrmarket_wait_for_events", _wait_for_nostr_events
     )
-    scheduled_tasks.extend([task1, task2, task3])
+
+    async def _health_monitor():
+        # start after the subscription is active
+        await asyncio.sleep(20)
+        await subscription_health_monitor(nostr_client)
+
+    task4 = create_permanent_unique_task(
+        "ext_nostrmarket_health_monitor", _health_monitor
+    )
+    scheduled_tasks.extend([task1, task2, task3, task4])