docs: add DEFAULT_RESULTS_LOOKBACK_DAYS environment variable

docs/reference/environment-variables.md

@@ -30,3 +30,20 @@ See [docs/user-guide/configuration.md](../user-guide/configuration.md#environmen
 - `SEARCH_HTTP_PORT` (default: 8001)
 - `TRADE_HTTP_PORT` (default: 8002)
 - `GETPRICE_HTTP_PORT` (default: 8003)
+
+### DEFAULT_RESULTS_LOOKBACK_DAYS
+
+**Type:** Integer
+**Default:** 30
+**Required:** No
+
+Number of calendar days to look back when querying `/results` endpoint without date filters.
+
+**Example:**
+```bash
+# Default to last 60 days
+DEFAULT_RESULTS_LOOKBACK_DAYS=60
+```
+
+**Usage:**
+When no `start_date` or `end_date` parameters are provided to `/results`, the endpoint returns data from the last N days (ending today).