Harden API input validation, error handling, and optimistic rollbacks

Add try/catch around `request.json()` in all POST routes to return a
400 instead of crashing on malformed bodies. Validate `type` as a
strict `"movie" | "tv"` enum in search, discover, import, and resolve
routes. Validate `tmdbId` as a positive integer. Add a
`SORT_BY_PATTERN` regex and page-range check (1–500) to the discover
route. Wrap all outbound TMDB calls in try/catch and return 502 on
failure so clients get a structured error rather than an unhandled
rejection.

Fix optimistic-update rollbacks in `use-title-actions`: capture
`prevStatus` and `prevWatches` before each mutation and restore both
atoms in the catch block for catchUp, handleMarkSeason,
handleUnmarkSeason, and single-episode toggle.

Fix a bug in `getContinueWatchingFeed` where the watchDateMap could
hold a stale date for episodes watched more than once; the map now
keeps the most-recent `watchedAt` per episode.

app/api/explore/discover/route.ts

@@ -5,6 +5,9 @@ import { isTmdbConfigured } from "@/lib/config";
 import { discover } from "@/lib/tmdb/client";
 import { tmdbImageUrl } from "@/lib/tmdb/image";
 
+const SORT_BY_PATTERN = /^[a-z_]+\.(asc|desc)$/;
+const MAX_PAGE = 500;
+
 export async function GET(req: NextRequest) {
   const session = await auth.api.getSession({
     headers: await headers(),
@@ -24,10 +27,36 @@ export async function GET(req: NextRequest) {
   }
 
   const { searchParams } = req.nextUrl;
-  const type = searchParams.get("type") === "tv" ? "tv" : "movie";
+  const rawType = searchParams.get("type");
+  if (rawType && rawType !== "movie" && rawType !== "tv") {
+    return NextResponse.json(
+      { error: "type must be movie or tv" },
+      { status: 400 },
+    );
+  }
+  const type = rawType === "tv" ? "tv" : "movie";
   const genre = searchParams.get("genre");
   const sortBy = searchParams.get("sort_by") || "popularity.desc";
-  const page = searchParams.get("page") || "1";
+  const pageRaw = searchParams.get("page") || "1";
+  const page = Number.parseInt(pageRaw, 10);
+
+  if (!Number.isInteger(page) || page < 1 || page > MAX_PAGE) {
+    return NextResponse.json(
+      { error: `page must be an integer between 1 and ${MAX_PAGE}` },
+      { status: 400 },
+    );
+  }
+
+  if (!SORT_BY_PATTERN.test(sortBy)) {
+    return NextResponse.json(
+      { error: "Invalid sort_by value" },
+      { status: 400 },
+    );
+  }
+
+  if (genre && !/^\d+(,\d+)*$/.test(genre)) {
+    return NextResponse.json({ error: "Invalid genre value" }, { status: 400 });
+  }
 
   const params: Record<string, string> = {
     sort_by: sortBy,
@@ -37,7 +66,15 @@ export async function GET(req: NextRequest) {
     params.with_genres = genre;
   }
 
-  const results = await discover(type, params, Number(page));
+  let results: Awaited<ReturnType<typeof discover>>;
+  try {
+    results = await discover(type, params, page);
+  } catch {
+    return NextResponse.json(
+      { error: "Failed to fetch discover results" },
+      { status: 502 },
+    );
+  }
 
   const filtered = results.results.filter((r) => r.poster_path);