fix: prevent sporadic SSH permission denied by validating key content

The root cause of sporadic "Permission denied (publickey)" errors was
that validateSshKey() only checked if the key file existed on disk,
never verifying its content matched the database. When keys were rotated
or updated, the stale file persisted and SSH used the wrong key.

Changes:
- validateSshKey() now refreshes key from DB and compares file content
- Server saved event detects private_key_id changes to invalidate mux
- PrivateKey storeInFileSystem() uses file locking to prevent races
- PrivateKey saved event auto-resyncs file on key content changes
- Enforces 0600 permissions on key files

Fixes coollabsio/coolify#7724

app/Models/Server.php

@@ -135,7 +135,7 @@ class Server extends BaseModel
             $server->forceFill($payload);
         });
         static::saved(function ($server) {
-            if ($server->privateKey?->isDirty()) {
+            if ($server->wasChanged('private_key_id') || $server->privateKey?->isDirty()) {
                 refresh_server_connection($server->privateKey);
             }
         });