feat: add Nostr zap support (NIP-57) for splitpayments

[hexdaemon]

Summary

• When splitting payments to LNURL/Lightning Address targets that support Nostr zaps (NIP-57), the extension now sends a signed zap request (kind 9734) so recipients are notified via Nostr
• Adds per-wallet Nostr private key settings (hex or nsec), encrypted at rest, using the built-in lnbits-extension-settings-btn-dialog component (matching the lnurlp extension pattern)
• Gracefully falls back to normal LNURL payment if no key is configured or the target doesn't support zaps

Changes

• helpers.py (new): parse_nostr_private_key() accepting hex or nsec format
• models.py: SplitPaymentSettings model with wallet-scoped nostr_private_key
• migrations.py: m005 creates settings table, m006 adds wallet column as primary key
• crud.py: Per-wallet settings CRUD with encrypt_internal_message/decrypt_internal_message
• views_api.py: GET/PUT/DELETE /api/v1/settings endpoints using require_admin_key
• tasks.py: Two-phase LNURL flow — handle() to get LnurlPayResponse, then custom callback with nostr query param for zap-capable targets; falls back to execute_pay_request() otherwise
• UI: Settings gear button inline with wallet dropdown, per-wallet key configuration

Test plan

• Configure a Nostr private key via the settings dialog for a wallet
• Send a payment to a wallet with LNURL/Lightning Address targets that support zaps
• Verify splitpayments: sent zap request for <target> appears in logs
• Verify zap receipt appears on the recipient's Nostr client
• Verify targets that don't support zaps still receive normal LNURL payments
• Verify switching wallets in the UI loads/saves separate settings per wallet
• Delete the key and verify fallback to non-zap payments

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