Consolidate the data directory mode into one owner (closes #288)

internal/datadir now owns the DATA_DIR mode as an exported constant,
DirPerm; internal/database consumes it in both places that create the
directory rather than keeping its own copy. datadir is the owner
because guarding and creating DATA_DIR is its whole purpose, and the
dependency runs the right way: datadir imports only the standard
library and the flock package, so database importing datadir adds no
cycle. A comment on DirPerm names it as the single source, chosen over
a source-scanning test because it survives refactoring and builds no
machinery that inspects the tree. The mode value is unchanged.

Model: opus-4-8

internal/database/webhook_db_manager.go

@@ -13,6 +13,7 @@ import (
 	"gorm.io/driver/sqlite"
 	"gorm.io/gorm"
 	"sneak.berlin/go/webhooker/internal/config"
+	"sneak.berlin/go/webhooker/internal/datadir"
 	"sneak.berlin/go/webhooker/internal/gormlog"
 	"sneak.berlin/go/webhooker/internal/logger"
 )
@@ -53,8 +54,9 @@ func NewWebhookDBManager(
 		log:     params.Logger.Get(),
 	}
 
-	// Create data directory if it doesn't exist
-	err := os.MkdirAll(m.dataDir, dataDirPerm)
+	// Create data directory if it doesn't exist. datadir.DirPerm is the
+	// single source of the directory mode; either package may run first.
+	err := os.MkdirAll(m.dataDir, datadir.DirPerm)
 	if err != nil {
 		return nil, fmt.Errorf(
 			"creating data directory %s: %w",