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>

docker-compose.yml

@@ -1,16 +1,18 @@
+name: canon
+
 services:
-  topdeck:
+  canon:
     build: .
-    image: topdeck:latest
-    container_name: topdeck
+    image: canon:latest
+    container_name: canon
     restart: unless-stopped
     ports:
       - "4242:4242"
     volumes:
-      - topdeck-data:/data
+      - canon-data:/data
     env_file:
       - path: .env
         required: false
 
 volumes:
-  topdeck-data:
+  canon-data: