exclude mutual funds from any server fetch - they do not have earnings

src/service.zig

@@ -325,6 +325,11 @@ pub const DataService = struct {
     /// Smart refresh: even if cache is fresh, re-fetches when a past earnings
     /// date has no actual results yet (i.e. results just came out).
     pub fn getEarnings(self: *DataService, symbol: []const u8) DataError!FetchResult(EarningsEvent) {
+        // Mutual funds (5-letter tickers ending in X) don't have quarterly earnings.
+        if (isMutualFund(symbol)) {
+            return .{ .data = &.{}, .source = .cached, .timestamp = std.time.timestamp() };
+        }
+
         var s = self.store();
         const today = fmt.todayDate();
 
@@ -715,4 +720,10 @@ pub const DataService = struct {
             std.Thread.sleep(10 * std.time.ns_per_s);
         }
     }
+
+    /// Mutual funds use 5-letter tickers ending in X (e.g. FDSCX, VSTCX, FAGIX).
+    /// These don't have quarterly earnings on Finnhub.
+    fn isMutualFund(symbol: []const u8) bool {
+        return symbol.len == 5 and symbol[4] == 'X';
+    }
 };