feat: Discovery alert filters + notification reasons (v0.9.6)

- Add 4th "Discovery" tab in Alert Filters for member/topic follow notifications,
  with per-source enable toggle, independent event-type filters, and per-entity
  mute chips (mute specific members/topics without unfollowing)
- Enrich notification event payloads with follow_mode, matched_member_name,
  matched_member_id, and matched_topic so each event knows why it was created
- Dispatcher branches on payload.source for member_follow/topic_follow events,
  checking source-level enabled toggle, per-event-type filters, and muted_ids/muted_tags
- Add _build_reason helper; ntfy messages append a "why" line (📌/👤/🏷)
- EventRow in notification history shows a small italic reason line
- Update How It Works: fix stale member/topic paragraph, add Discovery alerts item

Authored-by: Jack Levy

frontend/app/how-it-works/page.tsx

@@ -120,9 +120,12 @@ export default function HowItWorksPage() {
         </div>
 
         <p className="text-xs text-muted-foreground">
-          You can also follow <strong>members</strong> (alerts when they sponsor new bills) and{" "}
-          <strong>topics</strong> (alerts when new bills matching that topic are briefed).
-          Member and topic follows use the Follow mode filters.
+          You can also follow <strong>members</strong> and <strong>topics</strong>.
+          When a followed member sponsors a bill, or a new bill matches a followed topic, you&apos;ll
+          receive a <em>Discovery</em> alert. These have their own independent filter set in{" "}
+          <Link href="/notifications" className="text-primary hover:underline">Notifications → Alert Filters → Discovery</Link>.
+          By default, all followed members and topics trigger notifications — you can mute individual
+          ones without unfollowing them.
         </p>
       </Section>
 
@@ -176,6 +179,15 @@ export default function HowItWorksPage() {
             A private, tokenized RSS feed of all your bill alerts. Subscribe in any RSS reader
             (Feedly, NetNewsWire, etc.). Completely independent of ntfy.
           </Item>
+          <Item icon={Filter} color="bg-teal-100 text-teal-700 dark:bg-teal-900/30 dark:text-teal-400" title="Discovery alerts">
+            Member and topic follows generate Discovery alerts — separate from the bills you follow
+            directly. In{" "}
+            <Link href="/notifications" className="text-primary hover:underline">Alert Filters → Discovery</Link>,
+            you can enable or disable these independently, tune which event types trigger them, and
+            mute specific members or topics you&apos;d rather not hear about without unfollowing them.
+            Each notification also shows a &ldquo;why&rdquo; line so you always know which follow
+            triggered it.
+          </Item>
         </div>
       </Section>