Release-Ankündigungen + öffentliche Changelog-Seite

- Gitea-Release-Webhook: Releases werden archiviert (releases-Tabelle) und neue
  'published'-Releases als oranges 🚀-Embed mit Buttons angekündigt
- Release-Kanal auf der Setup-Seite wählbar (leer = Feature aus) + Test-Button
- /changelog: öffentliche Seite mit Tag-Chips, Pre-Release-Badge, Release-Notes
- /api/releases öffentlich; Updates/Redeliveries posten nicht doppelt

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/App.jsx

@@ -2,6 +2,7 @@ import { useEffect, useMemo, useState } from 'react';
 import { Routes, Route, NavLink, Navigate } from 'react-router-dom';
 import { apiGet } from './api.js';
 import Devlogs from './pages/Devlogs.jsx';
+import Changelog from './pages/Changelog.jsx';
 import Commits from './pages/Commits.jsx';
 import Settings from './pages/Settings.jsx';
 
@@ -69,6 +70,7 @@ export default function App() {
 
                 <nav className="nav-links">
                     <NavLink to="/devlogs">Devlog</NavLink>
+                    <NavLink to="/changelog">Changelog</NavLink>
                     {me.admin && <NavLink to="/commits">Commits</NavLink>}
                     {me.admin && <NavLink to="/settings">Setup</NavLink>}
                 </nav>
@@ -92,6 +94,7 @@ export default function App() {
                 <Routes>
                     <Route path="/" element={<Navigate to="/devlogs" replace />} />
                     <Route path="/devlogs" element={<Devlogs me={me} />} />
+                    <Route path="/changelog" element={<Changelog />} />
                     <Route path="/commits" element={<Commits me={me} />} />
                     <Route path="/settings" element={<Settings me={me} />} />
                     <Route path="*" element={<Navigate to="/devlogs" replace />} />