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'}

frontend/src/components/scene/DeepControlPanel.svelte

@@ -6,7 +6,8 @@
 
     let error = '';
     let challengeTargetId = '';
-    let challengeStakes = '';
+    let stakesSuccess = '';
+    let stakesFailure = '';
     let selectedObstacles = {};
     let captainSelectId = '';
 
@@ -21,10 +22,12 @@
             await apiRequest(`/game/${state.game.id}/player/${state.player.id}/challenge/create`, 'POST', {
                 target_player_id: challengeTargetId,
                 obstacle_ids: JSON.stringify(ids),
-                stakes: challengeStakes
+                stakes_success: stakesSuccess,
+                stakes_failure: stakesFailure
             });
             challengeTargetId = '';
-            challengeStakes = '';
+            stakesSuccess = '';
+            stakesFailure = '';
             selectedObstacles = {};
         } catch(e) {
             error = e.message;
@@ -88,7 +91,8 @@
                 </label>
             {/each}
         </div>
-        <input type="text" class="input-field" placeholder="Stakes (optional): what happens on success/failure?" bind:value={challengeStakes} style="width: 100%; margin-bottom: 0.5rem;">
+        <input type="text" class="input-field" placeholder="On success (optional): what they win…" bind:value={stakesSuccess} style="width: 100%; margin-bottom: 0.5rem;">
+        <input type="text" class="input-field" placeholder="On failure (optional): what it costs them…" bind:value={stakesFailure} style="width: 100%; margin-bottom: 0.5rem;">
         <button class="btn btn-primary btn-full" on:click={createChallenge}
                 disabled={!challengeTargetId || !Object.values(selectedObstacles).some(v => v)}>
             Call Challenge