chore: beta release v0.5.0-beta

- Bump version to 0.5.0-beta
- Add Apache-2.0 LICENSE (official text) + NOTICE
- Add Docker support: multi-stage Dockerfile + docker-compose
  * non-root user, tini PID 1, gunicorn + uvicorn workers
  * healthcheck, OCI labels, persistent /data volume
- Add GitHub Actions CI (lint/import-check + Docker smoke-test)
- Backend polish:
  * modern lifespan handler (replaces deprecated on_event)
  * explicit sort validation (400 instead of 500)
  * title min_length=1 schema validation
  * configurable data dir + http timeout via env
  * avg_rating helper extracted (DRY)
- Frontend polish:
  * beta badge in title, footer with live version
  * loading spinner on initial fetch
  * updated README, badges, config table, roadmap

app/static/index.html

@@ -15,7 +15,7 @@
       <div class="brand">
         <div class="logo">W</div>
         <div class="brand-text">
-          <h1>WatchStack</h1>
+          <h1>WatchStack <span class="version-badge">beta</span></h1>
           <p>Deine Watchlist für <strong>Bücher</strong>, <strong>Serien</strong> & mehr.</p>
         </div>
       </div>
@@ -74,7 +74,10 @@
 
     <!-- Listen-Grid -->
     <section id="grid" class="grid">
-      <!-- Cards per JS -->
+      <div class="grid-loading">
+        <div class="spinner"></div>
+        <span>Lade Watchlist…</span>
+      </div>
     </section>
 
     <div id="empty-state" class="empty hidden">
@@ -219,6 +222,11 @@
     </div>
   </div>
 
+  <footer class="footer">
+    <span>WatchStack <span id="version">…</span> · Apache 2.0</span>
+    <span>Made for readers &amp; binge-watchers</span>
+  </footer>
+
   <!-- ===================== Toast ===================== -->
   <div id="toast" class="toast" hidden></div>