ads: add self-hosted click + impression tracking to AdImage

Migration creates bb.banner_analytics (mirrors article_analytics: public
insert, admin read), applied separately to Supabase.

AdImage becomes a client component that:
- Fires one 'impression' event per render via IntersectionObserver at 50%
  visibility (avoids counting ads scrolled past at the fold).
- Fires a 'click' event on anchor click before the new tab opens.

Renders the same <a><AppImage/></a> as before — visually unchanged. Session
ID stored in sessionStorage as bb_session_id (random uuid, lazily created).
Both parents (SidebarAdStack, ArticleBody) were already 'use client', so no
boundary shifts. Tracking is best-effort and never blocks the user.

src/components/AdImage.tsx

@@ -1,4 +1,9 @@
+"use client";
+
+import { useEffect, useRef } from "react";
+import { usePathname } from "next/navigation";
 import AppImage from "@/components/ui/AppImage";
+import { createClient } from "@/lib/supabase/client";
 import type { Ad } from "@/lib/ads";
 
 const SIZE_DIM: Record<Ad["size"], { w: number; h: number }> = {
@@ -7,6 +12,16 @@ const SIZE_DIM: Record<Ad["size"], { w: number; h: number }> = {
   "728x90":  { w: 728, h: 90 },
 };
 
+function getSessionId(): string {
+  if (typeof window === "undefined") return "";
+  let sid = sessionStorage.getItem("bb_session_id");
+  if (!sid) {
+    sid = crypto.randomUUID();
+    sessionStorage.setItem("bb_session_id", sid);
+  }
+  return sid;
+}
+
 export default function AdImage({
   ad,
   className = "",
@@ -17,11 +32,60 @@ export default function AdImage({
   priority?: boolean;
 }) {
   const { w, h } = SIZE_DIM[ad.size];
+  const anchorRef = useRef<HTMLAnchorElement | null>(null);
+  const impressionFiredRef = useRef(false);
+  const pathname = usePathname();
+
+  useEffect(() => {
+    const el = anchorRef.current;
+    if (!el || impressionFiredRef.current) return;
+
+    const observer = new IntersectionObserver(
+      (entries) => {
+        for (const entry of entries) {
+          if (entry.isIntersecting && !impressionFiredRef.current) {
+            impressionFiredRef.current = true;
+            observer.disconnect();
+            void recordEvent("impression");
+            break;
+          }
+        }
+      },
+      { threshold: 0.5 }
+    );
+    observer.observe(el);
+    return () => observer.disconnect();
+    // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, [ad.src]);
+
+  async function recordEvent(eventType: "impression" | "click") {
+    try {
+      const supabase = createClient();
+      await supabase.from("banner_analytics").insert({
+        ad_src: ad.src,
+        ad_label: ad.label,
+        ad_size: ad.size,
+        event_type: eventType,
+        page: pathname || (typeof window !== "undefined" ? window.location.pathname : null),
+        session_id: getSessionId(),
+        referrer: typeof document !== "undefined" ? document.referrer || null : null,
+      });
+    } catch {
+      /* tracking is best-effort; never block the user */
+    }
+  }
+
+  function handleClick() {
+    void recordEvent("click");
+  }
+
   return (
     <a
+      ref={anchorRef}
       href={ad.click_url}
       target="_blank"
       rel="sponsored noopener noreferrer"
+      onClick={handleClick}
       aria-label={`Advertisement — ${ad.label}`}
       className={`block ad-creative ${className}`}
       data-ad-size={ad.size}