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>

cmd/canon/main.go

@@ -1,4 +1,4 @@
-// Command topdeck serves a personal, self-hosted collection of ranked
+// Command canon serves a personal, self-hosted collection of ranked
 // favorites.
 package main
 
@@ -16,12 +16,12 @@ import (
 	"syscall"
 	"time"
 
-	"github.com/cdnutter/topdeck/internal/auth"
-	"github.com/cdnutter/topdeck/internal/config"
-	"github.com/cdnutter/topdeck/internal/imgcache"
-	"github.com/cdnutter/topdeck/internal/providers"
-	"github.com/cdnutter/topdeck/internal/server"
-	"github.com/cdnutter/topdeck/internal/store"
+	"github.com/cdnutter/canon/internal/auth"
+	"github.com/cdnutter/canon/internal/config"
+	"github.com/cdnutter/canon/internal/imgcache"
+	"github.com/cdnutter/canon/internal/providers"
+	"github.com/cdnutter/canon/internal/server"
+	"github.com/cdnutter/canon/internal/store"
 )
 
 // version is set at build time with -ldflags "-X main.version=…".
@@ -29,7 +29,7 @@ var version = "dev"
 
 func main() {
 	level := slog.LevelInfo
-	if strings.EqualFold(os.Getenv("TOPDECK_LOG_LEVEL"), "debug") {
+	if strings.EqualFold(os.Getenv("CANON_LOG_LEVEL"), "debug") {
 		level = slog.LevelDebug
 	}
 	log := slog.New(slog.NewTextHandler(os.Stdout, &slog.HandlerOptions{Level: level}))
@@ -92,7 +92,7 @@ func run(log *slog.Logger) error {
 	}
 	errc := make(chan error, 1)
 	go func() {
-		log.Info("TopDeck is running", "addr", cfg.ListenAddr, "data", cfg.DataDir, "version", version)
+		log.Info("Canon is running", "addr", cfg.ListenAddr, "data", cfg.DataDir, "version", version)
 		errc <- httpSrv.ListenAndServe()
 	}()
 	select {
@@ -119,7 +119,7 @@ func bootstrapAdmin(ctx context.Context, cfg config.Config, st *store.Store, srv
 	}
 	if cfg.AdminUsername != "" && cfg.AdminPassword != "" {
 		if len(cfg.AdminPassword) < auth.MinPasswordLength {
-			return fmt.Errorf("TOPDECK_ADMIN_PASSWORD must be at least %d characters", auth.MinPasswordLength)
+			return fmt.Errorf("CANON_ADMIN_PASSWORD must be at least %d characters", auth.MinPasswordLength)
 		}
 		hash, err := auth.HashPassword(cfg.AdminPassword)
 		if err != nil {