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>

tests/conftest.py

@@ -0,0 +1,27 @@
+"""
+Stub out the nostrmarket root package and all LNbits dependencies so that
+nostr/* unit tests can run without the full LNbits environment.
+
+pytest walks up from tests/ and tries to import the parent __init__.py,
+which pulls in fastapi, lnbits, websocket, etc. We preemptively register
+the parent package as a simple module so that import never happens.
+"""
+
+import sys
+import types
+from pathlib import Path
+
+# Register 'nostrmarket' as an already-imported namespace package
+# pointing at the extension root, so pytest doesn't try to exec __init__.py
+_ext_root = Path(__file__).resolve().parent.parent
+_pkg = types.ModuleType("nostrmarket")
+_pkg.__path__ = [str(_ext_root)]
+_pkg.__package__ = "nostrmarket"
+sys.modules["nostrmarket"] = _pkg
+
+# Also ensure the nostr subpackage is importable
+_nostr_dir = _ext_root / "nostr"
+_nostr_pkg = types.ModuleType("nostrmarket.nostr")
+_nostr_pkg.__path__ = [str(_nostr_dir)]
+_nostr_pkg.__package__ = "nostrmarket.nostr"
+sys.modules["nostrmarket.nostr"] = _nostr_pkg