Add public feed, admin area, self-registration, visibility & hashtags

- Public feed (/) with infinite scroll via Intersection Observer
- Self-registration (/register)
- Admin area (/admin/entries, /admin/users) with user management
- journal_entries: visibility (public/private) + hashtags fields
- users: is_admin flag
- DB schema updated (recreate DB to apply)
- CI: run go test via docker run (golang:1.25-alpine) — fixes 'go not found'

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/cmd/server/main.go

@@ -49,8 +49,9 @@ func main() {
 	stopStore := db.NewStopStore(pool)
 	suggStore := db.NewSuggestionStore(pool)
 	journalStore := db.NewJournalStore(pool)
+	userStore := db.NewUserStore(pool)
 
-	router := api.NewRouter(authStore, tpStore, stopStore, suggStore, journalStore, uploadDir)
+	router := api.NewRouter(authStore, tpStore, stopStore, suggStore, journalStore, userStore, uploadDir)
 
 	srv := &http.Server{
 		Addr:         addr,