fix: add-invoice crash and update docs

A required field was not set and for some reason prior Pydantic versions
didn't notice at all.

closes #150

app/repositories/ln_impl/lnd_grpc.py

@@ -421,6 +421,7 @@ async def add_invoice_impl(
         # Can't use Invoice.from_lnd_grpc() here because
         # the response is not a standard invoice
         invoice = Invoice(
+            value_msat=value_msat,
             memo=memo,
             expiry=expiry,
             r_hash=response.r_hash.hex(),

app/routers/lightning.py

@@ -69,7 +69,12 @@ responses = {
     responses=responses,
 )
 async def addinvoice(
-    value_msat: int, memo: str = "", expiry: int = 3600, is_keysend: bool = False
+    value_msat: int = Query(description="The amount of msat of the invoice", ge=0),
+    memo: str = Query("", description="The memo of the invoice"),
+    expiry: int = Query(3600, description="Expiry time in seconds"),
+    is_keysend: bool = Query(
+        False, description="LND only: Whether this invoice is a keysend invoice."
+    ),
 ):
     try:
         return await add_invoice(memo, value_msat, expiry, is_keysend)