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/lib/validate.ts

@@ -1,8 +1,12 @@
 import type { Context } from "hono";
 
-/** Parse a route param as a positive integer id. Returns null if invalid. */
+/**
+ * Parse a route param as a positive integer id. Returns null if invalid.
+ * Strict: rejects mixed strings like "42abc" that Number.parseInt would
+ * accept — route params must be wholly numeric or the request is bad.
+ */
 export function parseId(raw: string | undefined): number | null {
-	if (!raw) return null;
+	if (!raw || !/^\d+$/.test(raw)) return null;
 	const n = Number.parseInt(raw, 10);
 	return Number.isFinite(n) && n > 0 ? n : null;
 }