feat(signer): migrate Nostr publishing off account.prvkey → resolve_for_wallet (#23)

Closes aiolabs/events#23. Pre-cascade prerequisite for aiolabs/lnbits#17
(signer abstraction phase 1), which lands an m002 startup job that
NULLs the legacy `accounts.prvkey` column. After this migration, the
events extension reads no plaintext nsec and works with any
NostrSigner backend (LocalSigner / RemoteBunkerSigner / ClientSideOnlySigner).

## What changed

### nostr_hooks.py — publish_or_delete_nostr_event

Was: pulled `(account.pubkey, account.prvkey)` from the wallet owner,
passed both to `publish_event_to_nostr`. Hard-skipped publish when
`account.prvkey` was None.

Now: calls `await resolve_for_wallet(event.wallet)` (the DRY helper
from aiolabs/lnbits#23 — wallet → account → signer → can_sign-check
in one call, returns None on any soft-fail). Passes the resolved
`NostrSigner` to the publisher. Soft-skip on None (wallet missing,
account unclassified, or ClientSideOnlySigner where the server has
no signing authority) — matching previous "no prvkey" behavior.

### nostr_publisher.py — publish_event_to_nostr

Was: accepted `(account_pubkey, account_prvkey)` and signed via a
local `sign_nostr_event` helper that called `coincurve.PrivateKey
.sign_schnorr` directly on the plaintext nsec.

Now: accepts `signer: NostrSigner`. Builds the unsigned event dict
(`kind`/`created_at`/`tags`/`content`), hands it to
`await signer.sign_event(...)`, reconstructs the local `NostrEvent`
model from the signed dict (`id`/`pubkey`/`sig` fields). The signer
backend (LocalSigner / RemoteBunkerSigner) is transparent.

Removed the `sign_nostr_event` helper entirely — the signer abstraction
handles all signing now.

Dropped the `coincurve` import; no direct crypto in this extension.

## Acceptance

- [x] keypair helper replaced (nostr_hooks no longer touches account.prvkey)
- [x] publish_event_to_nostr accepts NostrSigner instead of (pubkey, prvkey)
- [x] extension-local Schnorr code removed (sign_nostr_event gone)
- [x] re-grep `events/`: zero `account.prvkey` references
- [x] version bumped: 1.6.1-aio.3 → 1.6.1-aio.4

Manual smoke testing + tag + catalog entry follow the migration
landing; will run against the regtest stack with lnbits on
`issue-18-phase-2.3` (which validates both LocalSigner and
RemoteBunkerSigner signing paths end-to-end).

## Cross-references

- aiolabs/events#23 — issue this commit closes
- aiolabs/lnbits#17 — the cascading signer-abstraction PR
- aiolabs/lnbits#23 — the resolve_for_wallet helper this uses
- aiolabs/lnbits#26 — phase 2.3 (sign_event over bunker, validated against
  aiolabs/nsecbunkerd@fb1c239)
- aiolabs/lnbits#21 — umbrella audit identifying 5 affected extensions

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

nostr_publisher.py

@@ -1,8 +1,9 @@
 """
 NIP-52 calendar event publishing for the events extension.
 
-Builds NIP-52 calendar events from the Event model, signs them with the
-creator's Account keypair, and publishes via the NostrClient.
+Builds NIP-52 calendar events from the Event model, signs them via the
+core `NostrSigner` abstraction (backend-agnostic: LocalSigner,
+RemoteBunkerSigner, etc.), and publishes via the NostrClient.
 
 Kind 31922 is used for date-only events; kind 31923 (time-based) is used
 when event_start_date / event_end_date include a time component.
@@ -13,7 +14,7 @@ Reference: https://github.com/nostr-protocol/nips/blob/master/52.md
 import time
 from datetime import datetime, timezone
 
-import coincurve
+from lnbits.core.signers import NostrSigner
 from loguru import logger
 
 from .models import Event
@@ -142,23 +143,20 @@ def build_nip52_delete_event(event: Event, pubkey: str) -> NostrEvent:
     return nostr_event
 
 
-def sign_nostr_event(nostr_event: NostrEvent, private_key_hex: str) -> None:
-    """Sign a NostrEvent in-place using Schnorr signature."""
-    privkey = coincurve.PrivateKey(bytes.fromhex(private_key_hex))
-    sig = privkey.sign_schnorr(bytes.fromhex(nostr_event.id))
-    nostr_event.sig = sig.hex()
-
-
 async def publish_event_to_nostr(
     nostr_client,
     event: Event,
-    account_pubkey: str,
-    account_prvkey: str,
+    signer: NostrSigner,
     delete: bool = False,
 ) -> NostrEvent | None:
     """
     Build, sign, and publish a NIP-52 calendar event (or delete event).
 
+    Signing routes through the core `NostrSigner` abstraction —
+    `signer.pubkey` for the event identity, `await signer.sign_event(...)`
+    for the Schnorr signature. The signer backend (LocalSigner /
+    RemoteBunkerSigner) is transparent to this function.
+
     Returns the published NostrEvent for metadata storage, or None on failure.
     """
     if not nostr_client:
@@ -167,11 +165,25 @@ async def publish_event_to_nostr(
 
     try:
         if delete:
-            nostr_event = build_nip52_delete_event(event, account_pubkey)
+            nostr_event = build_nip52_delete_event(event, signer.pubkey)
         else:
-            nostr_event = build_nip52_event(event, account_pubkey)
+            nostr_event = build_nip52_event(event, signer.pubkey)
+
+        # Hand the unsigned event to the signer — it fills in `id`,
+        # `pubkey`, and `sig`. The signer's serialization rules match
+        # NIP-01 (same as the local `event_id` property uses), so the
+        # returned id matches what we'd have computed locally.
+        unsigned = {
+            "kind": nostr_event.kind,
+            "created_at": nostr_event.created_at,
+            "tags": nostr_event.tags,
+            "content": nostr_event.content,
+        }
+        signed = await signer.sign_event(unsigned)
+        nostr_event.id = signed["id"]
+        nostr_event.pubkey = signed["pubkey"]
+        nostr_event.sig = signed["sig"]
 
-        sign_nostr_event(nostr_event, account_prvkey)
         await nostr_client.publish_nostr_event(nostr_event)
 
         logger.info(