Add game-state rollback (Phase 1)

Full-state JSON snapshots let a scene be rolled back to an earlier
action. All game state lives in game_id-scoped tables, so a checkpoint
is just a serialized dump of the gameplay rows and rollback restores
it -- sidestepping deterministic replay (the deck shuffle is
materialized into state and captured verbatim).

- Checkpoint table + GameEvent.checkpoint_id (tri-state) +
  Game.rollback_timeline_version (models.py, Alembic migration).
- crud_rollback.py: serialize/apply/capture/seal-purge/rollback.
- Capture is driven by the broadcast middleware: snapshot per action
  while in the `scene` phase, seal+purge otherwise. Confined to the
  current scene; older scenes' checkpoints are purged at scene end.
- POST /game/{gid}/player/{pid}/rollback (routes_rollback.py),
  server-enforced for Admins and Deep players.
- EventLog.svelte: per-event rollback buttons + timeline reconciliation.
- Remove the orphaned /scene/rollback per-card-play undo (dead code
  since d7f8483, never wired up; superseded by full-state rollback).

Phase 1 truncates the future immediately; the greyed/undoable redo is
deferred to Phase 2.

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

frontend/src/components/EventLog.svelte

@@ -33,6 +33,12 @@
     let haveMore = true;
     let loadingOlder = false;
     let atBottom = true;
+    let error = '';
+    let timelineVersion = null;
+
+    // Rollback buttons appear during a scene, for Admins and Deep players (server-enforced too).
+    $: canRollback = state.game?.phase === 'scene'
+        && (state.player?.is_admin || state.player?.role === 'deep');
 
     $: mergePolled(state.events);
 
@@ -40,6 +46,14 @@
     // otherwise events the player paged back to would vanish as the window slides.
     async function mergePolled(polled) {
         if (!polled) return;
+        // A rollback truncates the log server-side; the version bump tells us to drop
+        // everything we'd accumulated (those future events no longer exist) and reseed.
+        const version = state.game?.rollback_timeline_version ?? 0;
+        if (timelineVersion !== null && version !== timelineVersion) {
+            events = [];
+            seenIds = new Set();
+        }
+        timelineVersion = version;
         if (events.length === 0) {
             haveMore = state.events_have_more ?? polled.length >= PAGE_SIZE;
         }
@@ -107,6 +121,21 @@
     function formatTime(ts) {
         return new Date(ts * 1000).toLocaleTimeString([], { hour: '2-digit', minute: '2-digit' });
     }
+
+    async function rollback(event) {
+        if (!confirm(`Roll the game back to just after "${event.message}"? Everything after it will be undone.`)) return;
+        error = '';
+        try {
+            await apiRequest(
+                `/game/${state.game.id}/player/${state.player.id}/rollback`,
+                'POST',
+                { checkpoint_id: event.checkpoint_id },
+            );
+            // The state-changed ping drives Dashboard to refetch the restored state.
+        } catch (e) {
+            error = e.message;
+        }
+    }
 </script>
 
 <div class="floating-event-log {open ? 'open' : ''}">
@@ -115,6 +144,9 @@
     </button>
 
     {#if open}
+        {#if error}
+            <div class="log-error">{error}</div>
+        {/if}
         <div class="log-content" bind:this={logEl} on:scroll={onScroll}>
             {#if events.length}
                 <div class="log-top">
@@ -131,6 +163,13 @@
                         <span class="log-icon">{KIND_ICONS[event.kind] || KIND_ICONS.info}</span>
                         <span class="log-message">{event.message}</span>
                         <span class="log-time">{formatTime(event.timestamp)}</span>
+                        {#if canRollback && event.checkpoint_id > 0}
+                            <button
+                                class="rollback-btn"
+                                title="Roll the game back to just after this event"
+                                on:click={() => rollback(event)}
+                            >↩</button>
+                        {/if}
                     </div>
                 {/each}
             {:else}
@@ -211,7 +250,7 @@
     }
     .log-entry {
         display: grid;
-        grid-template-columns: auto 1fr auto;
+        grid-template-columns: auto 1fr auto auto;
         gap: 8px;
         align-items: baseline;
         padding: 6px 8px;
@@ -222,6 +261,31 @@
         line-height: 1.35;
         color: color-mix(in srgb, var(--text) 85%, var(--text-muted));
     }
+    .rollback-btn {
+        background: transparent;
+        border: 1px solid var(--edge);
+        color: var(--text-muted);
+        border-radius: 6px;
+        padding: 0 6px;
+        font-size: 0.8rem;
+        line-height: 1.4;
+        cursor: pointer;
+        align-self: center;
+    }
+    .rollback-btn:hover {
+        background: color-mix(in srgb, var(--danger) 18%, transparent);
+        border-color: var(--danger);
+        color: var(--text);
+    }
+    .log-error {
+        margin: 6px 8px 0;
+        padding: 6px 10px;
+        background: color-mix(in srgb, var(--danger) 14%, transparent);
+        border: 1px solid var(--danger);
+        border-radius: var(--radius-md);
+        color: var(--text);
+        font-size: 0.8rem;
+    }
     .log-icon {
         font-size: 0.9rem;
     }