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

An audit of every file webhooker reads configuration or required state from found cases that carried on silently. A zero-length webhooker.db, and a missing or zero-length per-webhook database, now log the "created a new, empty database" warning naming the file; restart recovery opens every live webhook's database, checking under the manager's lock that it still exists, so a missing one is reported at start. The main database's open errors name webhooker.db, for the server and webhooker resetpw; resetpw refuses a zero-length webhooker.db. A directory in place of any database file or its -wal or -shm is refused naming it. The README says how each case is treated. Also closes #459.

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,
+	)
+}