Document the PHP api layer and fix what documenting it exposed

Writing down what each endpoint actually guarantees made the places it
didn't obvious — shell arguments reaching a crontab or a bash -c
unescaped, master.conf written without tmp+rename, and conf edits that
could be saved without ever being parsed.

Plugin/unraid/api/dryrun.php

@@ -1,4 +1,69 @@
 <?php
+// ═══════════════════════════════════════════════════════════════════════════════════════════════
+// PURPOSE
+//   Dry-run launcher. Starts one script through run_job.sh with --dry-run, so the scheduler
+//   page can show what a job would do without letting it do any of it.
+//
+// OPERATIONAL MODEL
+//   Identical to run.php but for two differences: --dry-run is added, and there is no
+//   already-running guard. The missing guard is deliberate — a dry run changes nothing, so
+//   there is no reason to refuse one while a real run is in progress, and the two write to
+//   the same log where their output is distinguishable by the dry-run banner.
+//
+//   Fire and forget. The child is nohup'd and detached, and the response returns immediately
+//   with ok:true meaning "launched", not "finished". Progress is followed through log.php.
+//
+// DESIGN PRINCIPLES
+//   Never executes the target script directly.
+//       Everything goes through run_job.sh, so a dry run gets the same locking, logging, stat
+//       file and exit handling as a scheduled run. A second execution path would be a second
+//       set of bugs.
+//
+//   --dry-run is added here, honoured there.
+//       This endpoint guarantees the flag is passed; whether a given script actually respects
+//       it is that script's contract. The scripts that accept the flag are the ones the UI
+//       offers the button for.
+//
+// OPERATIONAL SAFEGUARDS
+//   The job id is validated and then confirmed to exist.
+//       ^[a-zA-Z0-9_./\-]+\.sh$ plus an explicit '..' check — the slash must be permitted for
+//       Category/name.sh ids, so traversal is caught by its own test rather than by the
+//       character class. file_exists() then confirms the resolved path is a real script, so a
+//       well-formed id for a file that is not there fails before anything is spawned.
+//
+//   The location argument must be absolute and clean.
+//       Leading slash required, '..' rejected, control characters rejected — then passed as a
+//       single escapeshellarg'd --location= token.
+//
+//   Extra arguments cannot become shell syntax.
+//       Control characters are rejected, then the string is split on whitespace and each
+//       token escaped individually. The previous blocklist of metacharacters missed newlines,
+//       which would have terminated the command line and started a second one — a blocklist
+//       has to be right about every character, whereas escaping each token is right about all
+//       of them.
+//
+//   Every interpolated value is escaped, including the ones that are already validated.
+//       Runner path, id, script path and log path all go through escapeshellarg() even though
+//       none of them can currently carry a metacharacter. Validation and escaping guard
+//       different things, and the escaping is what stays correct if the validation is ever
+//       loosened.
+//
+//   Output is appended, never truncated.
+//       >> to the job's own log with stdin from /dev/null, so a dry run cannot consume the
+//       request's stdin or discard the history of previous runs.
+//
+// REQUEST
+//   POST id=<Category/name.sh>  [location=/absolute/path]  [extra_args=…]
+//
+// RESPONSE
+//   {"ok":true}                              launched — not completed
+//   {"ok":false,"error":"Invalid id"|"Script not found: …"|"Invalid location"
+//                      |"Invalid extra_args"}
+//
+// DEPENDS ON
+//   include/scheduler.php   vv_job_log_path(), vv_job_flags()
+//   run_job.sh              the single execution path for every job
+// ═══════════════════════════════════════════════════════════════════════════════════════════════
 header('Content-Type: application/json');
 require_once dirname(__DIR__) . '/include/scheduler.php';
 
@@ -25,8 +90,11 @@ if ($location && (!str_starts_with($location, '/') || str_contains($location, '.
     exit;
 }
 
+// Control characters are rejected outright — a newline would end the command line and start
+// a second one. Everything that survives is split on whitespace and escaped per token, so
+// the shell never parses any of it as syntax.
 $extra_args = trim($_POST['extra_args'] ?? '');
-if ($extra_args && preg_match('/[;&|`$<>\\\\"\']/', $extra_args)) {
+if ($extra_args !== '' && preg_match('/[\x00-\x1f\x7f]/', $extra_args)) {
     echo json_encode(['ok' => false, 'error' => 'Invalid extra_args']);
     exit;
 }
@@ -34,7 +102,10 @@ if ($extra_args && preg_match('/[;&|`$<>\\\\"\']/', $extra_args)) {
 $runner   = dirname(__DIR__) . '/run_job.sh';
 $flags    = vv_job_flags($id);
 $locArg   = $location   ? ' ' . escapeshellarg('--location=' . $location) : '';
-$extraStr = $extra_args ? ' ' . $extra_args : '';
+$extraStr = '';
+foreach (preg_split('/\s+/', $extra_args, -1, PREG_SPLIT_NO_EMPTY) as $tok) {
+    $extraStr .= ' ' . escapeshellarg($tok);
+}
 exec('nohup bash ' . escapeshellarg($runner) . ' ' . escapeshellarg($id) . ' ' . escapeshellarg($script) . ' --dry-run' . ($flags ? " $flags" : '') . ' --manual' . $locArg . $extraStr . ' >> ' . escapeshellarg($logFile) . ' 2>&1 </dev/null &');
 
 echo json_encode(['ok' => true]);