Cache the merged media catalog and trim card rendering

poc/src/media.ts

@@ -191,6 +191,13 @@ export class MediaService {
     /** The newest bot and scrape errors, capped. Surfaced in the Bot tab. */
     private readonly errors: MediaErrorEntry[] = [];
 
+    /**
+     * Merged-candidate cache. Every grid request otherwise rescans every scrape database; the
+     * key includes a revision of the scrape metadata and preview counters, so new pages, state
+     * changes, and fresh previews invalidate it. The TTL catches anything the revision misses.
+     */
+    private readonly candidateCache = new Map<string, { at: number; value: Array<{ meta: ScrapeMeta; row: MediaRow }> }>();
+
     /** One-shot maintenance: re-fetch previews for Discord rows closed out with HTTP errors. */
     private readonly recover = {
         state: "idle" as "idle" | "running" | "paused" | "done",
@@ -1168,6 +1175,14 @@ export class MediaService {
      * wins by default; an older row replaces it only when it is richer (downloaded, or with a
      * cached preview). Without this, a stopped or partial scrape hides usable previews.
      */
+    /** Cheap change signal for the candidate cache: ids, states, update times, preview counts. */
+    private catalogRevision(): string {
+        return this.engine
+            .list()
+            .map(meta => `${meta.id}:${meta.state}:${meta.updatedAt}:${meta.previewed ?? 0}`)
+            .join("|");
+    }
+
     private collectCandidates(options: {
         kind?: MediaKind;
         source?: string;
@@ -1195,6 +1210,54 @@ export class MediaService {
         after?: string;
         before?: string;
         sort?: MediaSort;
+    }): Array<{ meta: ScrapeMeta; row: MediaRow }> {
+        // Drop paging and empty values: every page of one filter set shares a single scan.
+        const shape: Record<string, unknown> = {};
+        for (const [name, value] of Object.entries(options)) {
+            if (name === "limit" || name === "offset") continue;
+            if (value === undefined || value === null || value === "") continue;
+            if (Array.isArray(value) && value.length === 0) continue;
+            shape[name] = value;
+        }
+
+        const key = `${this.catalogRevision()}\u0000${JSON.stringify(shape)}`;
+        const hit = this.candidateCache.get(key);
+        const now = Date.now();
+        if (hit && now - hit.at < 15_000) return hit.value;
+
+        const value = this.collectCandidatesUncached(options);
+        if (this.candidateCache.size > 60) this.candidateCache.clear();
+        this.candidateCache.set(key, { at: now, value });
+        return value;
+    }
+
+    private collectCandidatesUncached(options: {
+        kind?: MediaKind;
+        source?: string;
+        scrapeId?: string;
+        channelId?: string;
+        authorId?: string;
+        authorIds?: string[];
+        origin?: "dm" | "guild";
+        cachedOnly?: boolean;
+        exts?: string[];
+        excludeHosts?: string[];
+        excludeExts?: string[];
+        excludeProviders?: string[];
+        excludeSources?: string[];
+        includeProviders?: string[];
+        includeSources?: string[];
+        titleQuery?: string;
+        titleExact?: boolean;
+        sizeLessThan?: number;
+        sizeGreaterThan?: number;
+        lengthLessThan?: number;
+        lengthGreaterThan?: number;
+        aspect?: string;
+        resolution?: string;
+        after?: string;
+        before?: string;
+        sort?: MediaSort;
     }): Array<{ meta: ScrapeMeta; row: MediaRow }> {
         const candidates = new Map<string, { meta: ScrapeMeta; row: MediaRow }>();