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/templatetags/club_email.py

@@ -9,9 +9,27 @@ is relative.
 
 from django import template
 
+from club.models import Club
+
 register = template.Library()
 
 
+@register.filter
+def contrast_color(hex_color):
+    """Black or white, whichever reads on ``hex_color`` -- for a literal
+    fallback background (e.g. ``club.secondary_color|default:"#ec4899"``)
+    rather than a club's own colour, which already has a matching
+    ``primary_content_color``/``secondary_content_color`` computed for it.
+
+    Chaining this onto the *same* expression used for the background --
+    ``club.secondary_color|default:"#ec4899"|contrast_color`` -- rather than
+    hardcoding a second, independently-guessed text colour is what keeps the
+    two from drifting apart: a pale fallback and a dark one both get the
+    contrast Club._content_color_for would compute for them either way.
+    """
+    return Club._content_color_for(hex_color)
+
+
 @register.simple_tag(takes_context=True)
 def absolute_media_url(context, file_field):
     """An absolute URL for ``file_field`` (e.g. ``club.logo``), for use in an

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")