Name the database each command uses on stderr (closes #111)

scan, report and trees now print one line on stderr before anything
else, naming the database as an absolute path and saying whether the
path is the default or comes from SFDUPES_DATABASE; scan also says when
no database file exists yet. report and trees print it before opening
the database, so a missing-database error follows it. databasePath in
db.go now returns the absolute path and its source, and every command
uses that absolute path throughout, so errors and the lock file name it
too. README "Database" lists when scan writes each file, from the lock
file to the -wal and -shm files going away.

Unverified: the line on a terminal has no test of its own; it is
printed before any progress display, the same way in both cases.

Model: opus-5-5

db.go

@@ -91,13 +91,22 @@ var errSchemaVersion = errors.New("unsupported database schema version")
 var errScanRunning = errors.New("another scan is running")
 
 // databasePath resolves the database location: SFDUPES_DATABASE when
-// set and non-empty, the compiled-in default otherwise.
-func databasePath() string {
+// set and non-empty, the compiled-in default otherwise. It returns the
+// path made absolute against the working directory, and where the path
+// came from, "default" or "from SFDUPES_DATABASE", for the line each
+// subcommand prints first on stderr.
+func databasePath() (string, string, error) {
+	path, source := defaultDatabasePath, "default"
 	if p := os.Getenv(databaseEnv); p != "" {
-		return p
+		path, source = p, "from "+databaseEnv
 	}
 
-	return defaultDatabasePath
+	abs, err := filepath.Abs(path)
+	if err != nil {
+		return "", "", fmt.Errorf("resolve database path %s: %w", path, err)
+	}
+
+	return abs, source, nil
 }
 
 // scanParams are the connection parameters for scan: read-write, with