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

Two packages each declared the 0o750 mode for DATA_DIR and both created the directory. internal/datadir now exports DirPerm as the single definition, and internal/database uses it in both places it creates the directory. The value is unchanged, so existing deployments see no permission change. datadir owns it because guarding and creating DATA_DIR is that package's whole purpose and it imports nothing that would form a cycle.

Model: opus-4-8 (implementation and review); fable-5-1 (merge)

internal/datadir/lock.go

@@ -29,9 +29,11 @@ import (
 // process that was killed with SIGKILL blocks nothing.
 const LockFileName = "webhooker.lock"
 
-// dirPerm is the mode Acquire creates DATA_DIR with. It matches what
-// internal/database uses, since whichever runs first creates it.
-const dirPerm = 0o750
+// DirPerm is the mode DATA_DIR is created with. It is the single
+// source of that mode: internal/database consumes it rather than
+// keeping its own copy, so the two packages that both create the
+// directory cannot drift into disagreeing about its permissions.
+const DirPerm = 0o750
 
 // ErrLocked reports that another live process holds the data
 // directory. Callers that need to know whether a deployment is running
@@ -64,7 +66,7 @@ func Acquire(dir string) (*Lock, error) {
 		return nil, ErrNoDir
 	}
 
-	err := os.MkdirAll(dir, dirPerm)
+	err := os.MkdirAll(dir, DirPerm)
 	if err != nil {
 		return nil, fmt.Errorf(
 			"creating data directory %s: %w", dir, err,