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/register.html

@@ -0,0 +1,20 @@
+{{define "title"}}Registrieren — Journal{{end}}
+
+{{define "content"}}
+<main class="container" style="max-width:400px;margin-top:4rem">
+  <h1>Konto erstellen</h1>
+  {{if .Error}}<p class="error">{{.Error}}</p>{{end}}
+  <form method="post" action="/register">
+    <label>Benutzername</label>
+    <input type="text" name="username" value="{{.Username}}" required autofocus autocomplete="username">
+    <label>Passwort</label>
+    <input type="password" name="password" required autocomplete="new-password">
+    <label>Passwort bestätigen</label>
+    <input type="password" name="confirm" required autocomplete="new-password">
+    <button type="submit">Registrieren</button>
+  </form>
+  <p><small>Bereits registriert? <a href="/login">Anmelden</a></small></p>
+</main>
+{{end}}
+
+{{template "base" .}}