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/internal/api/templates/days.html

@@ -4,7 +4,10 @@
 <main class="container">
   <div class="page-header">
     <h1>REISEJOURNAL</h1>
-    <a href="/logout">[ Ausloggen ]</a>
+    <span>
+      {{if .IsAdmin}}<a href="/admin">[ Admin ]</a> · {{end}}
+      <a href="/logout">[ Ausloggen ]</a>
+    </span>
   </div>
   <form method="get" action="/days/redirect">
     <fieldset role="group">