Fix fee calculation and improve token spendability detection

- Fix critical fee calculation bug: Now gets exact melt quote before creating invoice
- Improve spent token detection: Only marks as spent with clear indicators
- Add spent field to decode endpoint response (always boolean)
- Add informative root endpoint with API documentation
- Update documentation examples to use cashuB format
- Install bolt11 library for proper Lightning invoice verification
- Enhanced error handling and logging throughout

This fixes the issue where users lost sats due to fee estimation errors
and ensures accurate token spendability detection.

README.md

@@ -34,7 +34,7 @@ Decode a Cashu token and return its content. Supports both v1 and v3 token forma
 **Request:**
 ```json
 {
-  "token": "cashuAeyJhbGciOi..."
+  "token": "cashuB..."
 }
 ```
 
@@ -47,7 +47,8 @@ Decode a Cashu token and return its content. Supports both v1 and v3 token forma
     "totalAmount": 21000,
     "numProofs": 3,
     "denominations": [1000, 10000, 10000],
-    "format": "cashuA"
+    "format": "cashuA",
+    "spent": false
   },
   "mint_url": "https://mint.azzamo.net"
 }
@@ -59,7 +60,7 @@ Redeem a Cashu token to a Lightning address. Lightning address is optional - if
 **Request:**
 ```json
 {
-  "token": "cashuAeyJhbGciOi...",
+  "token": "cashuB...",
   "lightningAddress": "user@ln.tips"
 }
 ```
@@ -67,7 +68,7 @@ Redeem a Cashu token to a Lightning address. Lightning address is optional - if
 **Request (using default address):**
 ```json
 {
-  "token": "cashuAeyJhbGciOi..."
+  "token": "cashuB..."
 }
 ```