Threading: Antworten (reply_to) + Focus-View

- entry: reply_to/reply_count/last_activity; reply_count bubbelt bis Root
- Hauptfeed nur Roots nach last_activity, Profil-Feed inkl. Antworten
- GET /entry/{pid}/thread (Ahnen+Antworten), GET /e/{pid} Focus-Seite
- Frontend: Antworten-Link im Feed, entry.html/entry.js Focus-Seite
- notes/migrations.md: ALTER TABLE fuer Prod

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

main.go

@@ -37,6 +37,8 @@ func routes() http.Handler {
 	// --- Entries ---
 	r.Get("/entry/feed/{page}", handleFeed)
 	r.Get("/entry/{pid}/votes", handleVotes)
+	r.Get("/entry/{pid}/thread", handleThread)
+	r.Get("/e/{pid}", serveEntryPage)
 
 	// --- User (öffentlich) ---
 	r.Get("/u/{username}", serveUserPage)