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>

.env.example

@@ -1,21 +1,21 @@
 # Copy to .env and fill in what you need. Everything here is optional.
 
-# Create the owner account on first start. If unset, TopDeck prints a one-time
+# Create the owner account on first start. If unset, Canon prints a one-time
 # setup code to the log and you create the account at http://<host>:4242/setup.
-TOPDECK_ADMIN_USERNAME=
-TOPDECK_ADMIN_PASSWORD=
+CANON_ADMIN_USERNAME=
+CANON_ADMIN_PASSWORD=
 
 # API keys. You can also enter these in Settings → Integrations; values set
 # here take precedence and are locked in the web UI.
-TOPDECK_TMDB_API_KEY=
-TOPDECK_TVDB_API_KEY=
-TOPDECK_TVDB_PIN=
-TOPDECK_IGDB_CLIENT_ID=
-TOPDECK_IGDB_CLIENT_SECRET=
+CANON_TMDB_API_KEY=
+CANON_TVDB_API_KEY=
+CANON_TVDB_PIN=
+CANON_IGDB_CLIENT_ID=
+CANON_IGDB_CLIENT_SECRET=
 
 # Behind a reverse proxy that terminates HTTPS:
-# TOPDECK_TRUST_PROXY=true     # honor X-Forwarded-For / -Proto / -Host
-# TOPDECK_SECURE_COOKIES=true  # always mark the session cookie Secure
+# CANON_TRUST_PROXY=true     # honor X-Forwarded-For / -Proto / -Host
+# CANON_SECURE_COOKIES=true  # always mark the session cookie Secure
 
-# TOPDECK_PORT=4242
-# TOPDECK_LOG_LEVEL=debug
+# CANON_PORT=4242
+# CANON_LOG_LEVEL=debug