Rename project from TopDeck to Canon

- Module github.com/cdnutter/canon, binary/command cmd/canon, Docker
  image and compose project "canon"
- Environment variables now use the CANON_ prefix
- Default site title is "<Your name>'s Canon" (or "Canon" with no name);
  a Site title set in Settings still overrides it
- Existing topdeck.db (and WAL files) is renamed to canon.db on start
- README: upgrade notes, including moving the old compose volume

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/templates/layout.html

@@ -39,7 +39,7 @@
   <div class="container footer-inner">
     <span>{{if .Site.OwnerName}}Curated by {{.Site.OwnerName}}{{else}}{{.Site.Title}}{{end}}</span>
     <span class="footer-sep" aria-hidden="true">·</span>
-    <span>Powered by <a href="https://git.iamchrisama.com/cdnutter/topdeck" rel="noopener">TopDeck</a></span>
+    <span>Powered by <a href="https://git.iamchrisama.com/cdnutter/canon" rel="noopener">Canon</a></span>
   </div>
 </footer>
 {{block "scripts" .}}{{end}}