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/admin/users.html

@@ -0,0 +1,53 @@
+{{define "admin_title"}}Benutzer verwalten — Admin{{end}}
+
+{{define "admin_content"}}
+<h1>Benutzer</h1>
+
+{{if .Error}}<p class="error">{{.Error}}</p>{{end}}
+
+<form method="post" action="/admin/users" style="display:flex;gap:1rem;align-items:flex-end;flex-wrap:wrap">
+  <div>
+    <label>Benutzername</label>
+    <input type="text" name="username" required autocomplete="off">
+  </div>
+  <div>
+    <label>Passwort</label>
+    <input type="password" name="password" required autocomplete="new-password">
+  </div>
+  <button type="submit">Anlegen</button>
+</form>
+
+<figure>
+<table>
+  <thead><tr><th>Benutzername</th><th>Admin</th><th>Erstellt</th><th></th></tr></thead>
+  <tbody>
+  {{range .Users}}
+  <tr>
+    <td>{{.Username}}</td>
+    <td>{{if .IsAdmin}}✓{{end}}</td>
+    <td><small>{{.CreatedAt.Format "2006-01-02"}}</small></td>
+    <td>
+      {{if ne .UserID $.User.UserID}}
+      <button class="btn-delete" data-url="/admin/users/{{.UserID}}" data-name="{{.Username}}">Löschen</button>
+      {{else}}
+      <small>(du)</small>
+      {{end}}
+    </td>
+  </tr>
+  {{end}}
+  </tbody>
+</table>
+</figure>
+{{end}}
+
+{{define "admin_scripts"}}
+<script>
+document.querySelectorAll('.btn-delete').forEach(function(btn) {
+  btn.addEventListener('click', function() {
+    if (!confirm('Benutzer "' + btn.dataset.name + '" löschen?')) return;
+    fetch(btn.dataset.url, {method: 'DELETE'})
+      .then(function() { window.location.reload(); });
+  });
+});
+</script>
+{{end}}