address audit findings: subtitle rescan decisions, scan limit, parseId, setup gate

worked through AUDIT.md. triage:
- finding 2 (subtitle rescan wipes decisions): confirmed. /:id/rescan now
  snapshots custom_titles and calls reanalyze() after the stream delete/
  insert, mirroring the review rescan flow. exported reanalyze + titleKey
  from review.ts so both routes share the logic.
- finding 3 (scan limit accepts NaN/negatives): confirmed. extracted
  parseScanLimit into a pure helper, added unit tests covering NaN,
  negatives, floats, infinity, numeric strings. invalid input 400s and
  releases the scan_running lock.
- finding 4 (parseId lenient): confirmed. tightened the regex to /^\d+$/
  so "42abc", "abc42", "+42", "42.0" all return null. rewrote the test
  that codified the old lossy behaviour.
- finding 5 (setup_complete set before jellyfin test passes): confirmed.
  the /jellyfin endpoint still persists url+key unconditionally, but now
  only flips setup_complete=1 on a successful connection test.
- finding 6 (swallowed errors): partial. the mqtt restart and version-
  fetch swallows are intentional best-effort with downstream surfaces
  (getMqttStatus, UI fallback). only the scan.ts db-update swallow was
  a real visibility gap — logs via logError now.
- finding 1 (auth): left as-is. redacting secrets on GET without auth
  on POST is security theater; real fix is an auth layer, which is a
  design decision not a bugfix. audit removed from the tree.
- lint fail on ffmpeg.test.ts: formatted.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/api/settings.ts

@@ -26,12 +26,15 @@ app.post("/jellyfin", async (c) => {
 	// { ok, saved, testError } shape to decide what message to show.
 	setConfig("jellyfin_url", url);
 	setConfig("jellyfin_api_key", apiKey);
-	setConfig("setup_complete", "1");
 
 	const result = await testJellyfin({ url, apiKey });
 
-	// Best-effort admin discovery only when the connection works; ignore failures.
+	// Only mark setup complete when the connection actually works. Setting
+	// setup_complete=1 on a failing test would let the user click past the
+	// wizard into an app that then dies on the first Jellyfin call.
 	if (result.ok) {
+		setConfig("setup_complete", "1");
+		// Best-effort admin discovery only when the connection works; ignore failures.
 		try {
 			const users = await getUsers({ url, apiKey });
 			const admin = users.find((u) => u.Name === "admin") ?? users[0];