Report or refuse each unusable file webhooker reads (closes #290)

Audit of the files webhooker reads configuration or required state
from. A missing or zero-length database is reported with the "created
a new, empty database" warning and its path: webhooker.db at start,
and a per-webhook database at the latest at the next start, since
restart recovery now opens every webhook's database. The main
database's open errors name webhooker.db
(#459). webhooker resetpw
refuses a zero-length webhooker.db as it refuses a missing one. A
directory in place of a database file or its -wal or -shm is refused,
naming it; beside a -shm directory SQLite opened the database
read-only without a word. The README says how each case is treated.

Model: opus-5-5

internal/resetpw/resetpw.go

@@ -308,7 +308,7 @@ func checkDataDir(dir string) error {
 
 	dbPath := filepath.Join(dir, database.MainDBFileName)
 
-	_, err = os.Stat(dbPath)
+	dbInfo, err := os.Stat(dbPath)
 
 	switch {
 	case errors.Is(err, fs.ErrNotExist):
@@ -319,6 +319,15 @@ func checkDataDir(dir string) error {
 		)
 	case err != nil:
 		return fmt.Errorf("checking %s: %w", dbPath, err)
+	case dbInfo.Size() == 0:
+		// SQLite opens a zero-length file as an empty database, so
+		// it holds no deployment either, and opening it would write
+		// an empty schema into it.
+		return fmt.Errorf(
+			"%w: %s is zero-length. The admin account is created by "+
+				"the first server start",
+			ErrNoDatabase, dbPath,
+		)
 	}
 
 	return nil

internal/resetpw/resetpw_test.go

@@ -377,6 +377,33 @@ func TestMissingDatabaseCreatesNothing(t *testing.T) {
 	)
 }
 
+// TestZeroLengthDatabaseCreatesNothing covers a webhooker.db left at
+// zero length, as a truncated copy leaves it. SQLite would open it as
+// an empty database, so it is refused like a missing one and left as
+// it is.
+func TestZeroLengthDatabaseCreatesNothing(t *testing.T) {
+	dir := t.TempDir()
+	t.Setenv("DATA_DIR", dir)
+
+	dbPath := filepath.Join(dir, database.MainDBFileName)
+	require.NoError(
+		t, os.WriteFile(dbPath, nil, database.SQLiteFilePerm),
+	)
+
+	code, _, stderr := run(t, newPassword+"\n", operatorUser)
+
+	require.Equal(t, exitFailure, code)
+	assert.Contains(t, stderr, dbPath)
+
+	entries, err := os.ReadDir(dir)
+	require.NoError(t, err)
+	assert.Len(t, entries, 1, "nothing may be created beside it")
+
+	info, err := os.Stat(dbPath)
+	require.NoError(t, err)
+	assert.Zero(t, info.Size(), "nothing may be written into it")
+}
+
 // TestUnknownUserFails states the decision: resetpw changes an
 // existing account's password and never creates an account. A typo in
 // the username must say so rather than quietly adding a second user.