Report a zero-length or missing database that starts empty (closes #290)

Audit of the configuration and state read from files. Three cases
carried on silently: a zero-length webhooker.db booted as a first
start without the "created a new, empty database" warning, and a
missing or zero-length per-webhook database was replaced by an empty
one with only an INFO line, losing that webhook's events and pending
deliveries unannounced. All three now log that warning with the
file's path; CreateDB, which makes a new webhook's file, does not.

The README now says how .env, the main, per-webhook and archive
databases treat an empty file, and that a damaged per-webhook database
fails that webhook alone.

Model: opus-5-5

internal/database/bootstrap_banner_test.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"context"
 	"log/slog"
+	"os"
 	"path/filepath"
 	"strings"
 	"testing"
@@ -119,3 +120,26 @@ func TestNewDatabase_IsLoggedWithItsPath(t *testing.T) {
 		t, second, created, "an existing database is not new",
 	)
 }
+
+// TestZeroLengthDatabase_IsLoggedAsNew covers what
+// https://git.eeqj.de/sneak/webhooker/issues/290 found: SQLite opens a
+// zero-length file as an empty database, so a start on one is a first
+// start, and it must say so exactly as a start with no file does.
+func TestZeroLengthDatabase_IsLoggedAsNew(t *testing.T) {
+	t.Parallel()
+
+	dir := t.TempDir()
+	path := filepath.Join(dir, database.MainDBFileName)
+	require.NoError(t, os.WriteFile(path, nil, database.SQLiteFilePerm))
+
+	var out bytes.Buffer
+
+	db, err := database.Open(dir, slog.New(slog.NewTextHandler(&out, nil)))
+	require.NoError(t, err)
+	require.NoError(t, db.Close())
+
+	assert.Contains(
+		t, out.String(),
+		`level=WARN msg="created a new, empty database" path=`+path,
+	)
+}