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/entries.html

@@ -0,0 +1,33 @@
+{{define "admin_title"}}Einträge verwalten — Admin{{end}}
+
+{{define "admin_content"}}
+<h1>Einträge</h1>
+<p><a href="/days">→ Neuen Eintrag anlegen (Tagesansicht)</a></p>
+
+{{if .Entries}}
+<figure>
+<table>
+  <thead><tr><th>Datum</th><th>Zeit</th><th>Titel</th><th>Sichtbarkeit</th><th>Hashtags</th></tr></thead>
+  <tbody>
+  {{range .Entries}}
+  <tr>
+    <td><a href="/days/{{.EntryDate}}">{{.EntryDate}}</a></td>
+    <td>{{.EntryTime}}</td>
+    <td>{{if .Title}}{{.Title}}{{else}}<small>—</small>{{end}}</td>
+    <td>
+      {{if eq .Visibility "public"}}
+        <span class="badge-public">öffentlich</span>
+      {{else}}
+        <small>privat</small>
+      {{end}}
+    </td>
+    <td><small>{{join .Hashtags ", "}}</small></td>
+  </tr>
+  {{end}}
+  </tbody>
+</table>
+</figure>
+{{else}}
+<p><small>// Noch keine Einträge</small></p>
+{{end}}
+{{end}}