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.go

@@ -6,6 +6,7 @@ import (
 	"database/sql"
 	"errors"
 	"fmt"
+	"os"
 	"path/filepath"
 
 	_ "modernc.org/sqlite"
@@ -18,7 +19,10 @@ type Store struct {
 }
 
 func Open(dataDir string) (*Store, error) {
-	path := filepath.Join(dataDir, "topdeck.db")
+	path := filepath.Join(dataDir, "canon.db")
+	if err := adoptLegacyDB(dataDir, path); err != nil {
+		return nil, err
+	}
 	dsn := fmt.Sprintf("file:%s?_pragma=journal_mode(WAL)&_pragma=foreign_keys(1)&_pragma=busy_timeout(5000)&_pragma=synchronous(NORMAL)&_txlock=immediate", path)
 	db, err := sql.Open("sqlite", dsn)
 	if err != nil {
@@ -35,6 +39,25 @@ func Open(dataDir string) (*Store, error) {
 
 func (s *Store) Close() error { return s.db.Close() }
 
+// adoptLegacyDB renames a database from before the project was called
+// Canon (topdeck.db, plus its WAL files) so upgrades keep their data.
+func adoptLegacyDB(dataDir, path string) error {
+	legacy := filepath.Join(dataDir, "topdeck.db")
+	if _, err := os.Stat(path); !errors.Is(err, os.ErrNotExist) {
+		return nil
+	}
+	if _, err := os.Stat(legacy); err != nil {
+		return nil
+	}
+	for _, suffix := range []string{"-wal", "-shm", ""} {
+		err := os.Rename(legacy+suffix, path+suffix)
+		if err != nil && !errors.Is(err, os.ErrNotExist) {
+			return fmt.Errorf("renaming %s to %s: %w", legacy+suffix, path+suffix, err)
+		}
+	}
+	return nil
+}
+
 // migrations are applied in order; the index into this slice (plus one) is
 // the schema version stored in PRAGMA user_version.
 var migrations = []string{