Fix Pay User lightning payment bugs

- Fix default amount showing fiat instead of sats when lightning payment selected
- Fix invoice response field name (bolt11 instead of payment_request)
- Fix NameError in payables/pay endpoint (wallet -> auth.user_id)
- Add get_user_wallet_settings_by_prefix() for truncated 8-char user IDs
- Update user-wallet endpoint to handle truncated IDs from Beancount accounts

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

crud.py

@@ -424,6 +424,26 @@ async def get_user_wallet_settings(user_id: str) -> Optional[UserWalletSettings]
     )
 
 
+async def get_user_wallet_settings_by_prefix(
+    user_id_prefix: str,
+) -> Optional[StoredUserWalletSettings]:
+    """
+    Get user wallet settings by user ID prefix (for truncated 8-char IDs from Beancount).
+
+    Beancount accounts use truncated user IDs (first 8 chars), but the database
+    stores full UUIDs. This function looks up by prefix to bridge the gap.
+    """
+    return await db.fetchone(
+        """
+        SELECT * FROM user_wallet_settings
+        WHERE id LIKE :prefix || '%'
+        LIMIT 1
+        """,
+        {"prefix": user_id_prefix},
+        StoredUserWalletSettings,
+    )
+
+
 async def update_user_wallet_settings(
     user_id: str, data: UserWalletSettings
 ) -> UserWalletSettings: