Add Discord-hosted preview caching (C11)

poc/src/media.ts

@@ -94,6 +94,7 @@ export interface MediaItemResult {
     kind: MediaKind;
     url: string;
     localUrl: string | null;
+    thumbUrl: string | null;
     provider: string | null;
     downloaded: boolean;
     createdAt: string;
@@ -141,6 +142,7 @@ export class MediaService {
             log: line => this.deps.log?.(line),
             download: {
                 maxFileBytes: number(process.env.VESKSLOP_MEDIA_MAX_FILE_BYTES, 50 * 1024 * 1024),
+                previewMaxBytes: number(process.env.VESKSLOP_MEDIA_PREVIEW_MAX_BYTES, 5 * 1024 * 1024),
                 minGapMs: number(process.env.VESKSLOP_MEDIA_DOWNLOAD_MIN_MS, 1000),
                 maxGapMs: number(process.env.VESKSLOP_MEDIA_DOWNLOAD_MAX_MS, 3000)
             }
@@ -323,6 +325,17 @@ export class MediaService {
         };
     }
 
+    /** Absolute path for a cached preview, or null. */
+    getMediaThumb(scrapeId: string, itemId: number): { path: string; contentType: string } | null {
+        const item = this.getMediaItem(scrapeId, itemId);
+        if (!item?.preview_hash) return null;
+
+        return {
+            path: join(this.rootDir, scrapeId, "thumbs", `${item.preview_hash}.webp`),
+            contentType: "image/webp"
+        };
+    }
+
     /** One media row inside a scrape, with its scrape and channel for context. */
     getMediaItem(scrapeId: string, itemId: number): (MediaRow & { scrapeId: string; channelId: string }) | null {
         const meta = this.engine.get(scrapeId);
@@ -363,6 +376,7 @@ export class MediaService {
                         kind: row.kind,
                         url: row.url,
                         localUrl: row.file_path ? `/api/media/${meta.id}/${row.id}/file` : null,
+                        thumbUrl: row.preview_hash ? `/api/media/${meta.id}/${row.id}/thumb` : null,
                         provider: row.provider,
                         downloaded: row.downloaded === 1,
                         createdAt: row.created_at