Split Deep challenge stakes into success and failure fields

The Deep now states what happens on success and on failure as two
separate fields when calling a Challenge, instead of one free-text
stakes blob. ChallengePanel shows them as "✅ On success" / "❌ On
failure"; the event log lists both.

- Challenge.stakes_success / stakes_failure (+ migration); the old
  single `stakes` column is kept (vestigial) so in-scene rollback
  snapshots taken before the upgrade still deserialize, and ChallengePanel
  falls back to it for any pre-existing challenge.
- create_challenge / route / DeepControlPanel updated; tests adjusted.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/components/scene/ChallengePanel.svelte

@@ -78,7 +78,13 @@
                 </div>
             {/if}
         </div>
-        {#if ch.stakes}
+        {#if ch.stakes_success}
+            <p class="info-text" style="margin: 0.5rem 0 0 0;"><strong>✅ On success:</strong> {ch.stakes_success}</p>
+        {/if}
+        {#if ch.stakes_failure}
+            <p class="info-text" style="margin: 0.25rem 0 0 0;"><strong>❌ On failure:</strong> {ch.stakes_failure}</p>
+        {/if}
+        {#if ch.stakes && !ch.stakes_success && !ch.stakes_failure}
             <p class="info-text" style="margin: 0.5rem 0 0 0;"><strong>Stakes:</strong> {ch.stakes}</p>
         {/if}
         {#if ch.challenge_type === 'pvp'}