feat(filter): filter on when a QSO was added to the log

After an import that inserted instead of updating, there was no way to
ask which records it created: an import of old contacts carries old QSO
dates, so it hides among them. created_at is the only column that knows,
and it was not filterable.

Listed as a date column so a bare YYYY-MM-DD compares on the date part,
like qso_date.

frontend/src/components/FilterBuilder.tsx

@@ -33,6 +33,8 @@ type FieldType = 'text' | 'number' | 'date' | 'adifdate';
 const FIELDS: { value: string; label: string; type: FieldType }[] = [
   { value: 'callsign', label: 'fltb.fCallsign', type: 'text' },
   { value: 'qso_date', label: 'fltb.fDate', type: 'date' },
+  // When the record entered the logbook — the way to isolate what an import added.
+  { value: 'created_at', label: 'fltb.fCreated', type: 'date' },
   { value: 'qso_date_off', label: 'fltb.fEndDate', type: 'date' },
   { value: 'band', label: 'fltb.fBand', type: 'text' },
   { value: 'band_rx', label: 'fltb.fRxBand', type: 'text' },