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>

internal/config/config.go

@@ -28,19 +28,19 @@ type Config struct {
 }
 
 func Load() Config {
-	port := env("TOPDECK_PORT", "4242")
+	port := env("CANON_PORT", "4242")
 	return Config{
-		ListenAddr:       env("TOPDECK_LISTEN", ":"+port),
-		DataDir:          env("TOPDECK_DATA_DIR", "./data"),
-		AdminUsername:    env("TOPDECK_ADMIN_USERNAME", ""),
-		AdminPassword:    env("TOPDECK_ADMIN_PASSWORD", ""),
-		SecureCookies:    envBool("TOPDECK_SECURE_COOKIES", false),
-		TrustProxy:       envBool("TOPDECK_TRUST_PROXY", false),
-		TMDBKey:          env("TOPDECK_TMDB_API_KEY", ""),
-		TVDBKey:          env("TOPDECK_TVDB_API_KEY", ""),
-		TVDBPin:          env("TOPDECK_TVDB_PIN", ""),
-		IGDBClientID:     env("TOPDECK_IGDB_CLIENT_ID", ""),
-		IGDBClientSecret: env("TOPDECK_IGDB_CLIENT_SECRET", ""),
+		ListenAddr:       env("CANON_LISTEN", ":"+port),
+		DataDir:          env("CANON_DATA_DIR", "./data"),
+		AdminUsername:    env("CANON_ADMIN_USERNAME", ""),
+		AdminPassword:    env("CANON_ADMIN_PASSWORD", ""),
+		SecureCookies:    envBool("CANON_SECURE_COOKIES", false),
+		TrustProxy:       envBool("CANON_TRUST_PROXY", false),
+		TMDBKey:          env("CANON_TMDB_API_KEY", ""),
+		TVDBKey:          env("CANON_TVDB_API_KEY", ""),
+		TVDBPin:          env("CANON_TVDB_PIN", ""),
+		IGDBClientID:     env("CANON_IGDB_CLIENT_ID", ""),
+		IGDBClientSecret: env("CANON_IGDB_CLIENT_SECRET", ""),
 	}
 }