Added a "Published in the last X days" filter, applied by the backend

GET /api/opportunities takes released_from, an RFC 3339 time, and lists
only the opportunities whose latest release is at or after it. It works
with paging and with changed_after, which keeps the same batch either
way; an invalid value is 400.

The frontend's filter defaults to 7 days, from midnight UK time, and
sends it with every load and catch-up, so a year of opportunities isn't
downloaded to show a week's. Changing the days loads the list again;
Clear filters goes back to 7.

internal/api/common.go

@@ -3,6 +3,7 @@ package api
 import (
 	"fmt"
 	"strconv"
+	"time"
 )
 
 // intParam parses the query parameter text as an integer from low to high,
@@ -21,3 +22,21 @@ func intParam(text string, fallback, low, high int) (int, error) {
 
 	return number, nil
 }
+
+// timeParam parses the query parameter name's text as an RFC 3339 time, or
+// returns the zero time if it is empty. Its error is meant for the client.
+func timeParam(name, text string) (time.Time, error) {
+	if text == "" {
+		return time.Time{}, nil
+	}
+
+	when, err := time.Parse(time.RFC3339, text)
+	if err != nil {
+		return time.Time{}, fmt.Errorf(
+			"%s must be an RFC 3339 date and time, e.g. 2026-10-12T00:00:00+01:00",
+			name,
+		)
+	}
+
+	return when, nil
+}