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

scan.go

@@ -91,9 +91,11 @@ type fileMeta struct {
 // least one other file shares are ever hashed: a size-unique file
 // cannot be a duplicate. A file of headTailMin or more gets its content
 // hash only when its size, head, and tail match another file's. Flag
-// parsing and the at-least-one-operand check are done by cobra. The
-// scan holds the lock on the database for its whole run, so a second
-// scan fails before it walks the filesystem or opens the database.
+// parsing and the at-least-one-operand check are done by cobra. Before
+// anything else, the scan names the database on stderr, saying whether
+// it is new. The scan holds the lock on the database for its whole
+// run, so a second scan fails before it walks the filesystem or opens
+// the database.
 // Errors are returned rather than exiting, so that the deferred close —
 // which takes the database out of WAL mode — always runs, and the lock
 // is released after it. When ctx is cancelled, as by the SIGINT or
@@ -104,12 +106,26 @@ type fileMeta struct {
 func runScan(ctx context.Context, roots []string, workers int,
 	oneFS bool,
 ) error {
-	roots, err := resolveRoots(roots)
+	dbPath, source, err := databasePath()
 	if err != nil {
 		return err
 	}
 
-	dbPath := databasePath()
+	// A database file missing now is created by lockScanDatabase below.
+	what := "database"
+
+	_, err = os.Stat(dbPath)
+	if errors.Is(err, fs.ErrNotExist) {
+		what = "new database"
+	}
+
+	fmt.Fprintf(os.Stderr, "scan: %s %s (%s)\n", what, escapePath(dbPath),
+		source)
+
+	roots, err = resolveRoots(roots)
+	if err != nil {
+		return err
+	}
 
 	lock, err := lockScanDatabase(dbPath)
 	if err != nil {