feat: global activity feed + session digest

Ops "Executions" tab showed raw target UUIDs, alphabetical (not
recency) order, and a stale status vocabulary from an earlier schema
iteration — never actually usable as a live "what's happening" view.
Replaced with a new recency-ordered /api/v1/activity/recent endpoint
and matching table (human-readable action summaries, risk/status
badges, duration, inline error preview).

Also added /api/v1/activity/session/{id} + a collapsible SessionDigest
panel in the chat rail, answering "what did this session actually do"
(executions by status, entities touched, knowledge written) — the
missing piece for proactive outcome reporting to be visible in the UI,
not just in the chat transcript.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

web/src/pages/Chat.svelte

@@ -2,6 +2,7 @@
   import { messages, streaming, sendMessage, cancelStream, error } from '$lib/stores/chat'
   import SessionRail from '$lib/components/SessionRail.svelte'
   import SessionGraph from '$lib/components/SessionGraph.svelte'
+  import SessionDigest from '$lib/components/SessionDigest.svelte'
   import ToolCallGroup from '$lib/components/ToolCallGroup.svelte'
   import InlineApproval from '$lib/components/InlineApproval.svelte'
   import { Button } from '$lib/components/ui/button'
@@ -188,8 +189,11 @@
             : 'bg-border group-hover/rz:bg-primary/50'}"
         ></span>
       </button>
-      <div class="min-w-0 flex-1">
-        <SessionGraph />
+      <div class="flex min-w-0 flex-1 flex-col">
+        <SessionDigest />
+        <div class="min-h-0 flex-1">
+          <SessionGraph />
+        </div>
       </div>
     </div>
   {/if}