Add missing getblockhash method to DogecoinClient for late payment scanning

The crypto watcher's late payment scanner was failing with:
'DogecoinClient' object has no attribute 'getblockhash'

Added getblockhash() method to both DogecoinClient and MockDogecoinClient:
- Uses standard Bitcoin RPC getblockhash call with block height parameter
- Enables proper Dogecoin late payment detection in production
- Required for crypto watcher to scan for missed payments

Critical for production Dogecoin support.

make_post_sell/lib/crypto_clients.py

@@ -310,6 +310,10 @@ class DogecoinClient:
         """Get current block height."""
         return self._call("getblockcount")
 
+    def getblockhash(self, block_height: int) -> str:
+        """Get block hash for a specific block height."""
+        return self._call("getblockhash", [block_height])
+
     def getnetworkinfo(self) -> Dict[str, Any]:
         """Get network status information."""
         return self._call("getnetworkinfo")
@@ -435,6 +439,10 @@ class MockDogecoinClient:
     def getblockcount(self) -> int:
         return 5500000  # Mock block height
 
+    def getblockhash(self, block_height: int) -> str:
+        """Mock block hash for testing."""
+        return f"mock_block_hash_{block_height}"
+
     def getnetworkinfo(self) -> Dict[str, Any]:
         return {
             "version": 1140200,