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/database.go

@@ -17,12 +17,12 @@ import (
 	"gorm.io/gorm"
 	"sneak.berlin/go/webhooker/internal/banner"
 	"sneak.berlin/go/webhooker/internal/config"
+	"sneak.berlin/go/webhooker/internal/datadir"
 	"sneak.berlin/go/webhooker/internal/gormlog"
 	"sneak.berlin/go/webhooker/internal/logger"
 )
 
 const (
-	dataDirPerm       = 0750
 	randomPasswordLen = 16
 	sessionKeyLen     = 32
 )
@@ -185,7 +185,9 @@ func (d *Database) connect() error {
 // caller's decision.
 func (d *Database) connectTo(dataDir string) error {
 	// Ensure the data directory exists before opening the database.
-	err := os.MkdirAll(dataDir, dataDirPerm)
+	// datadir.DirPerm is the single source of the directory mode; this
+	// package creates the directory too, since either may run first.
+	err := os.MkdirAll(dataDir, datadir.DirPerm)
 	if err != nil {
 		return fmt.Errorf(
 			"creating data directory %s: %w",