feat(tournaments): add partnership tracking and tournament management

Implement comprehensive partnership tracking system and tournament
management features including round-robin, single/double elimination,
and Swiss system pairings.

Changes:
- Add partnership_games and partnership_stats tables
- Implement PartnershipTracker service for recording partnership data
- Add tournament generator with multiple formats
- Create tournament management actions and views
- Add acceptance tests for partnership tracking
- Fix PartnershipTracker to use single snapshot per player per match
- Fix round-robin test to query by round_number instead of hardcoded ID

Note: All 8 acceptance tests now passing

app/templates/admin/tournaments/new.html.erb

@@ -0,0 +1,40 @@
+<h1>Create New Tournament</h1>
+
+<form action="/admin/tournaments" method="post">
+  <input type="hidden" name="_csrf_token" value="<%= csrf_token %>">
+
+  <div>
+    <label for="name">Tournament Name *</label>
+    <input type="text" id="name" name="name" required placeholder="e.g., Spring Championship">
+  </div>
+
+  <div>
+    <label for="description">Description</label>
+    <textarea id="description" name="description" rows="3" placeholder="Optional description or rules..."></textarea>
+  </div>
+
+  <div>
+    <label for="event_date">Date/Time</label>
+    <input type="datetime-local" id="event_date" name="event_date" value="<%= Time.now.strftime('%Y-%m-%dT%H:%M') %>">
+  </div>
+
+  <div>
+    <label for="format">Tournament Format</label>
+    <select id="format" name="format">
+      <option value="single_elim">Single Elimination</option>
+      <option value="double_elim">Double Elimination</option>
+      <option value="round_robin">Round Robin</option>
+      <option value="swiss">Swiss System</option>
+    </select>
+  </div>
+
+  <div>
+    <label for="max_participants">Max Participants (teams)</label>
+    <input type="number" id="max_participants" name="max_participants" min="2" max="32" placeholder="Optional">
+    <small>Leave blank for no limit</small>
+  </div>
+
+  <button type="submit">Create Tournament</button>
+</form>
+
+<p><a href="/admin/tournaments">Cancel</a></p>