merge: SP5 live-tail — pub/sub + 3 stream endpoints + frontend tail integration

Brings the SP5 live-tail implementation into main:

Backend
  * EventBus (cyclone.pubsub): per-kind fan-out with drop-oldest overflow
  * FastAPI lifespan initializes the bus + db once per process
  * store.add() publishes claim_written / remittance_written /
    activity_recorded events on every batch write
  * GET /api/{claims,remittances,activity}/stream: NDJSON snapshot +
    live subscription + 15s idle heartbeat
  * EventBus.unsubscribe() lets the tail loop release its queue on
    client disconnect (no queue leak per open stream)

Frontend
  * src/lib/tail-stream.ts: streamTail() async-generator over fetch
  * src/store/tail-store.ts: zustand with FIFO cap 10k per slice
  * src/hooks/useTailStream.ts: connecting/live/reconnecting/stalled/error/closed
    state machine with 1→2→4→8→16→30s backoff
  * src/hooks/useMergedTail.ts: base + tail merge with filter
  * src/components/TailStatusPill.tsx: badge + Reconnect button
  * Claims, Remittances, ActivityLog pages wired to the tail

Tests
  * 437 backend tests pass (was 418 before SP5)
  * 154 frontend tests pass (was 124)
  * npm run typecheck clean
  * end-to-end smoke: open /api/claims/stream, POST 837, see new claims
    arrive in real time without refresh

# Conflicts:
#	src/pages/ActivityLog.tsx
#	src/pages/Remittances.test.tsx
#	src/pages/Remittances.tsx

src/pages/ActivityLog.tsx

@@ -1,6 +1,9 @@
 import { useCallback, useMemo } from "react";
 import { useSearchParams } from "react-router-dom";
 import { useActivity } from "@/hooks/useActivity";
+import { useTailStream } from "@/hooks/useTailStream";
+import { useMergedTail } from "@/hooks/useMergedTail";
+import { TailStatusPill } from "@/components/TailStatusPill";
 import { ActivityFeed } from "@/components/ActivityFeed";
 import { ActivityFilters, type SinceValue } from "@/components/ActivityFilters";
 import { Skeleton } from "@/components/ui/skeleton";
@@ -69,12 +72,17 @@ export function ActivityLog() {
   });
 
   const allItems = data?.items ?? [];
+
+  // SP5: live-tail wiring. `useMergedTail` appends tail-arriving events
+  // to the base query result, then we apply the same client-side
+  // multi-kind filter so live events respect the active filter set.
+  const { status: tailStatus, lastEventAt: tailLastEventAt, forceReconnect } =
+    useTailStream("activity");
+  const merged = useMergedTail("activity", allItems);
   const items = useMemo(() => {
-    // Server already returned a single-kind slice when `apiKind` is set;
-    // otherwise apply the (multi-)kind filter locally.
-    if (selectedKinds.length <= 1) return allItems;
-    return allItems.filter((a) => selectedKinds.includes(a.kind));
-  }, [allItems, selectedKinds]);
+    if (selectedKinds.length <= 1) return merged;
+    return merged.filter((a) => selectedKinds.includes(a.kind));
+  }, [merged, selectedKinds]);
 
   const writeParams = useCallback(
     (mutate: (next: URLSearchParams) => void) => {
@@ -152,6 +160,16 @@ export function ActivityLog() {
       ) : null}
 
       <div className="surface rounded-xl p-6">
+        <div className="flex flex-wrap items-center gap-3 mb-4">
+          {/* SP5: live-tail status pill, right-aligned in the toolbar. */}
+          <div className="ml-auto">
+            <TailStatusPill
+              status={tailStatus}
+              lastEventAt={tailLastEventAt}
+              onReconnect={forceReconnect}
+            />
+          </div>
+        </div>
         {isLoading ? (
           <div className="space-y-2">
             {Array.from({ length: 5 }).map((_, i) => (