Fix #215-api: reject malformed multipart burn_after_reads with 400

handleCreatePasteMultipart silently dropped non-numeric
burn_after_reads values (err == nil guard), so burn_after_reads=abc
created a 201 paste with NO burn budget. Now returns 400 with the
error+code shape ({'code':'burn_invalid'}) matching the JSON path,
where an invalid value fails unmarshal into the int field. An empty
value stays 'unset' like the JSON path's omitempty. Verified live:
burn_after_reads=abc -> 400 burn_invalid; =2 -> 201.

internal/api/attachments.go

@@ -186,9 +186,18 @@ func (a *apiServer) handleCreatePasteMultipart(w http.ResponseWriter, r *http.Re
 		case "burn_after_read":
 			p.BurnAfterRead = v == "true" || v == "1" || v == "on"
 		case "burn_after_reads":
-			if n, err := strconv.Atoi(v); err == nil {
-				p.BurnAfterReads = &n
+			if v == "" { // absent value: unset, like the JSON path's omitempty
+				continue
 			}
+			n, err := strconv.Atoi(v)
+			if err != nil {
+				// malformed burn_after_reads is a 400, not a silently-dropped
+				// budget: same 400-with-code shape the JSON path uses for
+				// invalid values (which fail unmarshal into the int field).
+				writeErrCode(w, 400, "burn_invalid", "burn_after_reads must be a positive number")
+				return
+			}
+			p.BurnAfterReads = &n
 		}
 	}