Rename TMDB_API_KEY to TMDB_API_READ_ACCESS_TOKEN and add optional base URL overrides

Renames the env var for clarity since it holds a read access token, not an API
key. Adds optional TMDB_API_BASE_URL and TMDB_IMAGE_BASE_URL env vars for
advanced users. Centralizes image URL construction into a shared tmdbImageUrl()
helper, replacing hardcoded URLs across all components.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/(pages)/dashboard/page.tsx

@@ -13,6 +13,7 @@ import { useCallback, useEffect, useState } from "react";
 import { DashboardSkeleton } from "@/components/skeletons";
 import { StatsSummary } from "@/components/stats-summary";
 import { TitleCard } from "@/components/title-card";
+import { tmdbImageUrl } from "@/lib/tmdb/image";
 import { useSession } from "@/lib/auth/client";
 
 interface ContinueWatchingItem {
@@ -270,11 +271,9 @@ function FeedSection({
 }
 
 function ContinueWatchingCard({ item }: { item: ContinueWatchingItem }) {
-  const stillUrl = item.nextEpisode?.stillPath
-    ? `https://image.tmdb.org/t/p/w500${item.nextEpisode.stillPath}`
-    : item.title.backdropPath
-      ? `https://image.tmdb.org/t/p/w500${item.title.backdropPath}`
-      : null;
+  const stillUrl =
+    tmdbImageUrl(item.nextEpisode?.stillPath ?? null, "w500") ??
+    tmdbImageUrl(item.title.backdropPath ?? null, "w500");
   const progress =
     item.totalEpisodes > 0
       ? (item.watchedEpisodes / item.totalEpisodes) * 100