fix: better error handling for public key constraint

- Add try/catch in claimUsername to handle UNIQUE constraint
- Return meaningful error: 'This public key has already claimed a different username'
- Enable observability logs for better debugging

wrangler.toml

@@ -39,7 +39,7 @@ command = ""
 
 [observability]
 [observability.logs]
-enabled = false
+enabled = true
 head_sampling_rate = 1
 invocation_logs = true
 persist = true