Add comprehensive documentation for withdrawal fix

WITHDRAWAL_FIX.md

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+# Withdrawal Token Persistence Fix
+
+This document describes the fix for the admin panel withdrawal feature that was causing coin loss due to non-persistent cashu tokens.
+
+## Problem
+
+The original admin panel withdrawal feature had a critical issue:
+- Generated cashu tokens were only displayed momentarily in the UI
+- If the modal was closed accidentally, the token was lost forever
+- No record of pending withdrawals was maintained
+- This resulted in actual coin loss for administrators
+
+## Solution
+
+### 1. Token Persistence
+- Added `PendingWithdrawal` database model to store all withdrawal tokens
+- Every withdrawal is now automatically saved to the database
+- Tokens are never lost, even if the UI is closed accidentally
+
+### 2. Pending Withdrawals Display
+- New "Pending Withdrawals" section in the admin panel
+- Shows all unclaimed withdrawal tokens with details:
+  - Amount and currency unit
+  - Mint URL
+  - Creation timestamp
+  - Auto-send status
+  - Lightning address (if auto-sent)
+- Easy copy-to-clipboard functionality for tokens
+- Ability to mark withdrawals as claimed when used
+
+### 3. Automatic Lightning Address Sending
+- Optional auto-send feature for withdrawals
+- If `RECEIVE_LN_ADDRESS` is configured and auto-send is enabled:
+  - Tokens are automatically sent to the Lightning address
+  - No manual token handling required
+  - Withdrawal is marked as completed automatically
+- Fallback to manual token storage if auto-send fails
+
+### 4. Enhanced UI/UX
+- Checkbox option to enable auto-send during withdrawal
+- Better feedback messages for successful operations
+- Real-time updates of pending withdrawals list
+- Improved error handling and user notifications
+
+## Database Changes
+
+### New Table: `pending_withdrawals`
+```sql
+CREATE TABLE pending_withdrawals (
+    id INTEGER PRIMARY KEY,
+    token TEXT NOT NULL,
+    amount INTEGER NOT NULL,
+    unit TEXT NOT NULL,
+    mint_url TEXT NOT NULL,
+    created_at TIMESTAMP NOT NULL,
+    claimed BOOLEAN NOT NULL DEFAULT FALSE,
+    auto_sent BOOLEAN NOT NULL DEFAULT FALSE,
+    ln_address TEXT,
+    notes TEXT
+);
+```
+
+## API Changes
+
+### Enhanced `/admin/withdraw` endpoint
+- New optional parameter: `auto_send_to_ln: bool`
+- Returns additional fields:
+  - `withdrawal_id`: Database ID of the stored withdrawal
+  - `auto_sent`: Whether the token was automatically sent
+  - `ln_address`: Lightning address used (if auto-sent)
+  - `amount_received`: Amount received by Lightning address
+
+### New endpoint: `/admin/api/mark-withdrawal-claimed/{withdrawal_id}`
+- POST endpoint to mark a withdrawal as claimed
+- Prevents accidental reuse of tokens
+- Helps track withdrawal status
+
+### New partial: `/admin/partials/pending-withdrawals`
+- HTMX-powered component for displaying pending withdrawals
+- Auto-refreshes when new withdrawals are created
+- Provides interactive token management
+
+## Configuration
+
+### Environment Variables
+The fix uses existing configuration:
+- `RECEIVE_LN_ADDRESS`: Lightning address for auto-send functionality
+- `ADMIN_PASSWORD`: Required for admin panel access
+
+### Settings
+Auto-send functionality respects the existing `receive_ln_address` setting that can be configured through the admin panel settings interface.
+
+## Usage
+
+### Manual Withdrawal (Default)
+1. Click "💸 Withdraw Balance" in admin panel
+2. Select mint and currency
+3. Enter withdrawal amount
+4. Click "Withdraw"
+5. Token is generated and stored in pending withdrawals
+6. Copy token from the result or from pending withdrawals list
+7. Use token in your cashu wallet
+8. Mark as claimed when used (optional)
+
+### Auto-Send Withdrawal
+1. Ensure `RECEIVE_LN_ADDRESS` is configured in settings
+2. Click "💸 Withdraw Balance" in admin panel
+3. Select mint and currency
+4. Enter withdrawal amount
+5. Check "Auto-send to Lightning address" checkbox
+6. Click "Withdraw"
+7. Funds are automatically sent to your Lightning address
+8. Withdrawal is marked as completed automatically
+
+## Migration
+
+The database migration `123abc456def_add_pending_withdrawals_table.py` will be automatically applied when the application starts, creating the new `pending_withdrawals` table.
+
+## Benefits
+
+1. **No More Coin Loss**: All withdrawal tokens are permanently stored
+2. **Better UX**: Clear visibility of all pending withdrawals
+3. **Automation**: Optional auto-send reduces manual token handling
+4. **Audit Trail**: Complete history of all withdrawals
+5. **Recovery**: Ability to retrieve tokens even after UI accidents
+6. **Flexibility**: Choice between manual tokens and automatic Lightning sends
+
+## Backward Compatibility
+
+This fix is fully backward compatible:
+- Existing withdrawal functionality continues to work
+- No breaking changes to existing APIs
+- New features are opt-in
+- Database migration is automatic and safe