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>

views_api.py

@@ -2113,7 +2113,7 @@ async def api_pay_user(
     if "meta" not in entry:
         entry["meta"] = {}
     entry["meta"]["payment-method"] = data.payment_method
-    entry["meta"]["paid-by"] = wallet.wallet.user
+    entry["meta"]["paid-by"] = auth.user_id
     if data.txid:
         entry["meta"]["txid"] = data.txid
 
@@ -2174,10 +2174,24 @@ async def api_get_user_wallet(
     user_id: str,
     auth: AuthContext = Depends(require_super_user),
 ) -> dict:
-    """Get user's wallet settings (super user only)"""
+    """Get user's wallet settings (super user only)
 
+    Supports both full UUIDs and truncated 8-char IDs (from Beancount accounts).
+    """
+    from .crud import get_user_wallet_settings_by_prefix
+
+    # First try exact match
     user_wallet = await get_user_wallet(user_id)
-    if not user_wallet:
+
+    # If not found and user_id looks like a truncated ID (8 chars), try prefix match
+    if not user_wallet or not user_wallet.user_wallet_id:
+        if len(user_id) <= 8:
+            stored_wallet = await get_user_wallet_settings_by_prefix(user_id)
+            if stored_wallet and stored_wallet.user_wallet_id:
+                user_wallet = stored_wallet
+                user_id = stored_wallet.id  # Use the full ID
+
+    if not user_wallet or not user_wallet.user_wallet_id:
         return {"user_id": user_id, "user_wallet_id": None}
 
     # Get invoice key for the user's wallet (needed to generate invoices)