Let eligible referees self-serve sign-up for games needing one

Creating (or re-teaming) a home game for a club-managed team now auto-
invites every eligible referee (teams.RefereeProfile) via a new
RefereeSignup model, notifying them the same way news/events already do.
They see it as its own distinct row on the mobile Calendar (own accent
colour) and can accept or decline right there -- accepting routes through
the existing capacity-checked assign_referee (assigned_by=None marks it
self-service), so it lands as a real EventReferee row with no separate sync
step. The desktop referee-management screen and event detail page both
surface pending invites and flag self-signed-up referees distinctly from
admin assignments.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ECGMEwrc2k4D8VQuwjstj9

assets/mobile.css

@@ -73,6 +73,14 @@
     --color-ice: #14b8e8;
     --color-ice-ink: #04212c;
 
+    /* Referee sign-up accent -- deliberately distinct from game/training/other's
+       club-red/info-blue/warn-amber (mobile/_calendar_row.html) so a "needs a
+       referee" row stands out from the member's own RSVP rows at a glance. */
+    --color-referee: #7c4dff;
+    --color-referee-bg: #f1ecff;
+    --color-referee-border: #d9caff;
+    --color-referee-text: #5b2fd6;
+
     --color-row-sel: #fff7f8;
     --color-row-focus: #f4f9ff;
     --color-row-warn: #fffdf6;
@@ -393,6 +401,12 @@
         border: 1px solid #dde1e7;
     }
 
+    .pill-referee {
+        background: var(--color-referee-bg);
+        color: var(--color-referee-text);
+        border: 1px solid var(--color-referee-border);
+    }
+
     /* --- Person switcher chips (Home's own -- see mobile/templates/mobile/home.html) --- */
 
     .person-chip {