fix(chat): thread query returned OLDEST 300, hiding new messages past the cap

THE disappearing-messages root cause (found via THREAD.length=300 vs 351 on server).
messages.thread and threadByConversation did 'ORDER BY created_at ASC LIMIT 300' —
the oldest 300. Once a DM/group passed 300 messages, every newer message was silently
dropped from the fetch, so anything sent after that point 'disappeared' (persisted
server-side, never returned to the client). Now: inner 'ORDER BY created_at DESC LIMIT'
takes the NEWEST N, outer ASC presents them oldest-first. Cap raised 300 -> 500.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server/repos.js

@@ -204,11 +204,15 @@ const messages = {
   attachmentsForDm: (teamId, a, b) => db.prepare(`SELECT at.id, at.name, at.mime, at.size, m.created_at FROM messages m JOIN attachments at ON at.id=m.attachment_id WHERE m.team_id=? AND m.conversation_id IS NULL AND ((m.sender_id=? AND m.recipient_id=?) OR (m.sender_id=? AND m.recipient_id=?)) AND m.deleted=0 ORDER BY m.created_at DESC`).all(teamId, a, b, b, a),
   linksForConversation: (teamId, conversationId) => db.prepare("SELECT body, created_at FROM messages WHERE team_id=? AND conversation_id=? AND deleted=0 AND body LIKE '%http%' ORDER BY created_at DESC").all(teamId, conversationId),
   linksForDm: (teamId, a, b) => db.prepare("SELECT body, created_at FROM messages WHERE team_id=? AND conversation_id IS NULL AND ((sender_id=? AND recipient_id=?) OR (sender_id=? AND recipient_id=?)) AND deleted=0 AND body LIKE '%http%' ORDER BY created_at DESC").all(teamId, a, b, b, a),
-  // Full 1:1 (DM) thread between two users (both directions), oldest first.
-  thread: (teamId, a, b, limit = 300) =>
-    db.prepare(`SELECT * FROM messages WHERE team_id=? AND conversation_id IS NULL
+  // Full 1:1 (DM) thread between two users (both directions). Take the NEWEST `limit` messages
+  // (inner DESC), then present them oldest-first. The old plain "ASC LIMIT" returned the OLDEST 300
+  // and silently dropped everything newer once a thread passed 300 — so new messages "disappeared".
+  thread: (teamId, a, b, limit = 500) =>
+    db.prepare(`SELECT * FROM (
+        SELECT * FROM messages WHERE team_id=? AND conversation_id IS NULL
           AND ((sender_id=? AND recipient_id=?) OR (sender_id=? AND recipient_id=?))
-      ORDER BY created_at ASC LIMIT ?`).all(teamId, a, b, b, a, limit),
+          ORDER BY created_at DESC LIMIT ?
+      ) ORDER BY created_at ASC`).all(teamId, a, b, b, a, limit),
   markRead: (teamId, recipientId, senderId) =>
     db.prepare('UPDATE messages SET read_at=? WHERE team_id=? AND conversation_id IS NULL AND recipient_id=? AND sender_id=? AND read_at IS NULL')
       .run(now(), teamId, recipientId, senderId),
@@ -217,8 +221,10 @@ const messages = {
     db.prepare('SELECT * FROM messages WHERE team_id=? AND conversation_id IS NULL AND (sender_id=? OR recipient_id=?) ORDER BY created_at DESC LIMIT ?')
       .all(teamId, userId, userId, limit),
   // Group conversation helpers.
-  threadByConversation: (conversationId, limit = 300) =>
-    db.prepare('SELECT * FROM messages WHERE conversation_id=? ORDER BY created_at ASC LIMIT ?').all(conversationId, limit),
+  threadByConversation: (conversationId, limit = 500) =>
+    db.prepare(`SELECT * FROM (
+        SELECT * FROM messages WHERE conversation_id=? ORDER BY created_at DESC LIMIT ?
+      ) ORDER BY created_at ASC`).all(conversationId, limit),
   lastInConversation: (conversationId) =>
     db.prepare('SELECT * FROM messages WHERE conversation_id=? ORDER BY created_at DESC LIMIT 1').get(conversationId),
   unreadInConversation: (conversationId, userId, since) =>