fix(database): prevent command injection in healthcheck via CMD exec-form

Replace CMD-SHELL string interpolation with CMD exec-form arrays in
healthcheck configs for PostgreSQL, Dragonfly, KeyDB, and ClickHouse.

CMD-SHELL passes the string to /bin/sh -c, allowing command injection
through user-controlled fields (username, password, dbname). CMD
exec-form bypasses the shell entirely — each value is a discrete argv
element.

Fixes GHSA-gvc4-f276-r88p.

Adds regression tests covering semicolon, pipe, backtick, $(),
background operator, redirect, newline, and null-byte injection vectors.

app/Actions/Database/StartKeydb.php

@@ -109,7 +109,7 @@ class StartKeydb
                     ],
                     'labels' => defaultDatabaseLabels($this->database)->toArray(),
                     'healthcheck' => [
-                        'test' => "keydb-cli --pass {$this->database->keydb_password} ping",
+                        'test' => ['CMD', 'keydb-cli', '--pass', (string) $this->database->keydb_password, 'ping'],
                         'interval' => '5s',
                         'timeout' => '5s',
                         'retries' => 10,