Fix mismatched fallback text colour on the initials badge/button

The initials badge and reset button hardcoded two independent
fallback literals -- a background (#ec4899 / #0ea5e9) and a text
colour (#ffffff) -- that were only ever chosen together for a club's
own colour via Club._content_color_for. #ffffff on #ec4899 or
#0ea5e9 actually contrasts worse than black by the same WCAG formula
the app already uses elsewhere (verified: 6.4:1 vs 3.3:1, and 7.6:1
vs 2.8:1). Added a contrast_color filter so the text colour is always
derived from whatever background hex is actually in play -- real
club colour or fallback alike -- instead of a second, independently
guessed literal that can silently drift out of sync with the first.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

club/tests.py

@@ -1258,6 +1258,17 @@ class ClubBrandingModelTests(TestCase):
     def test_no_colour_means_no_contrast_colour(self):
         self.assertEqual(Club(primary_color="").primary_content_color, "")
 
+    def test_contrast_color_filter_matches_the_same_algorithm(self):
+        # HTML emails apply this to a literal fallback background (e.g.
+        # club.secondary_color|default:"#ec4899") rather than a club's own
+        # colour, so it can't be exercised through primary_content_color --
+        # but it must still agree with it for a colour a club actually set.
+        from club.templatetags.club_email import contrast_color
+
+        self.assertEqual(contrast_color("#ec4899"), Club(secondary_color="#ec4899").secondary_content_color)
+        self.assertEqual(contrast_color("#fef08a"), "#000000")
+        self.assertEqual(contrast_color("#1e40af"), "#ffffff")
+
     def test_a_colour_must_be_a_hex_value(self):
         club = Club(name="Ajax United", primary_color="blue")