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/store/store_test.go

@@ -4,6 +4,8 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"os"
+	"path/filepath"
 	"strings"
 	"testing"
 )
@@ -177,3 +179,31 @@ func TestCustomList(t *testing.T) {
 		t.Errorf("movie aspect = %q", m.Aspect())
 	}
 }
+
+func TestAdoptLegacyDB(t *testing.T) {
+	dir := t.TempDir()
+	s, err := Open(dir)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, err := s.CreateList(context.Background(), List{Title: "Keep me", Category: "movie", MaxItems: 5}); err != nil {
+		t.Fatal(err)
+	}
+	s.Close()
+	// Pretend this data dir predates the rename.
+	for _, suffix := range []string{"", "-wal", "-shm"} {
+		os.Rename(filepath.Join(dir, "canon.db"+suffix), filepath.Join(dir, "topdeck.db"+suffix))
+	}
+	s, err = Open(dir)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer s.Close()
+	lists, err := s.Lists(context.Background(), true)
+	if err != nil || len(lists) != 1 || lists[0].Title != "Keep me" {
+		t.Fatalf("legacy data not adopted: %v %v", lists, err)
+	}
+	if _, err := os.Stat(filepath.Join(dir, "topdeck.db")); !os.IsNotExist(err) {
+		t.Errorf("topdeck.db should have been renamed")
+	}
+}