perf(web-player): serve YouTube artwork through a caching proxy

The browser used to pull every cover straight from Google, so a search grid
plus infinite scroll fired dozens of cross-origin image requests and started
coming back as 429. Covers now go through /api/youtube/thumbnail, which
resizes the upstream URL, caches the bytes in memory, collapses duplicate
in-flight fetches, caps the concurrency against Google and backs off when it
answers 429. Responses are immutable for a month and revalidate with an ETag,
and serving them same-origin also lets the accent extractor read the pixels.

web-player/src/lib/server/config.ts

@@ -51,3 +51,30 @@ export const apiConfig = {
 		return (env.API_BASE_URL ?? '').replace(/\/+$/, '');
 	}
 };
+
+export const thumbnailConfig = {
+	get maxAgeSeconds() {
+		return positiveInt(env.THUMBNAIL_CACHE_MAX_AGE_SECONDS, 30 * DAY_SECONDS);
+	},
+	get failureCacheSeconds() {
+		return positiveInt(env.THUMBNAIL_FAILURE_CACHE_SECONDS, 300);
+	},
+	get memoryCacheBytes() {
+		return positiveInt(env.THUMBNAIL_CACHE_BYTES, 128 * 1024 * 1024);
+	},
+	get maxConcurrentFetches() {
+		return positiveInt(env.THUMBNAIL_MAX_CONCURRENT_FETCHES, 4);
+	},
+	get fetchTimeoutMs() {
+		return positiveInt(env.THUMBNAIL_FETCH_TIMEOUT_MS, 8000);
+	},
+	get cooldownSeconds() {
+		return positiveInt(env.THUMBNAIL_COOLDOWN_SECONDS, 30);
+	},
+	get maxCooldownSeconds() {
+		return positiveInt(env.THUMBNAIL_MAX_COOLDOWN_SECONDS, 300);
+	},
+	get maxSize() {
+		return positiveInt(env.THUMBNAIL_MAX_SIZE, 720);
+	}
+};