feat(api+broker+web): write-time mention fan-out via notification table

Phase 1 of v0.3.0 — replaces the regex-on-decoded-ciphertext scan
in /v1/notifications and the dashboard MentionsSection with reads
from a new mesh.notification table populated at write time.

Schema 0025: mesh.notification (id, mesh_id, topic_id, message_id,
recipient_member_id, sender_member_id, kind, created_at, read_at)
with a unique (message_id, recipient) so a re-fanned message yields
one row per recipient. Backfills existing v0.2.0 messages by
regex-matching the (still-base64-plaintext) bodies — guarded with
a base64 + length check so binary ciphertext doesn't crash the
migration.

Writers (POST /v1/messages + broker appendTopicMessage) now
extract @-mentions from either an explicit `mentions: string[]`
on the request OR a regex over the base64 plaintext (transitional
fallback). Targets are intersected with the mesh roster + capped
at 32 per message. Web chat panel sends the explicit array now so
it keeps working after phase 2 lands.

Readers switch to JOIN-on-notification:
  /v1/notifications      — table-backed, supports ?unread=1
  POST /v1/notifications/read  — new, mark by ids or all-up-to
  MentionsSection (RSC) — same JOIN, returns readAt for each row

GET /v1/notifications also gains a read_at field per row so a
future bell UI can show unread vs read.

Once per-topic encryption (phase 2) lands, the regex fallback
becomes a no-op for v2 messages — clients MUST send `mentions`,
which they already do.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/web/src/modules/mesh/topic-chat-panel.tsx

@@ -414,6 +414,21 @@ export function TopicChatPanel({
     [draft, mentionState],
   );
 
+  // Extract @-mention tokens from the draft body so the server can
+  // populate mesh.notification rows without having to read the
+  // ciphertext (forward-compat with v0.3.0 per-topic encryption).
+  // Capped at 16 to bound notification fan-out.
+  const extractMentions = (text: string): string[] => {
+    const found = new Set<string>();
+    const re = /(^|[^A-Za-z0-9_-])@([A-Za-z0-9_-]{1,64})(?=$|[^A-Za-z0-9_-])/g;
+    let m: RegExpExecArray | null;
+    while ((m = re.exec(text)) !== null) {
+      found.add(m[2]!.toLowerCase());
+      if (found.size >= 16) break;
+    }
+    return [...found];
+  };
+
   const send = async () => {
     const text = draft.trim();
     if (!text) return;
@@ -421,10 +436,16 @@ export function TopicChatPanel({
     setError(null);
     try {
       const { ciphertext, nonce } = encodeOutgoing(text);
+      const mentions = extractMentions(text);
       const res = await fetch("/api/v1/messages", {
         method: "POST",
         headers,
-        body: JSON.stringify({ topic: topicName, ciphertext, nonce }),
+        body: JSON.stringify({
+          topic: topicName,
+          ciphertext,
+          nonce,
+          ...(mentions.length > 0 ? { mentions } : {}),
+        }),
       });
       if (!res.ok) {
         const body = await res.text().catch(() => "");