feat: add worker threads, persistent Python sidecar, graceful shutdown, and architectural improvements

- Graceful shutdown: SIGTERM/SIGINT handlers drain HTTP, stop workers, close DB
- Thumbnail caching: disk-cached thumbnails with immutable Cache-Control headers
- Worker thread pool: Piscina offloads Sharp processing off the main event loop
- Persistent Python dispatcher: pre-imports ML libraries, eliminates cold-start latency
- Tool page registry: declarative tool-to-component mapping replaces 750-line switch
- File store cleanup: remove dead derived fields, stable files array reference
- Job persistence: progress written to SQLite jobs table, stale jobs recovered on startup

apps/api/src/lib/file-storage.ts

@@ -1,5 +1,5 @@
 import { randomUUID } from "node:crypto";
-import { mkdir, unlink, writeFile } from "node:fs/promises";
+import { mkdir, readFile, unlink, writeFile } from "node:fs/promises";
 import { extname, join } from "node:path";
 import { env } from "../config.js";
 
@@ -49,3 +49,39 @@ export async function deleteStoredFile(storedName: string): Promise<void> {
 export function getStoredFilePath(storedName: string): string {
   return join(env.FILES_STORAGE_PATH, storedName);
 }
+
+// ── Thumbnail cache ──────────────────────────────────────────────────
+
+const THUMB_DIR = ".thumbs";
+let thumbDirReady = false;
+
+async function ensureThumbDir(): Promise<void> {
+  if (thumbDirReady) return;
+  await mkdir(join(env.FILES_STORAGE_PATH, THUMB_DIR), { recursive: true });
+  thumbDirReady = true;
+}
+
+function thumbPath(storedName: string): string {
+  return join(env.FILES_STORAGE_PATH, THUMB_DIR, `${storedName}.thumb.jpg`);
+}
+
+export async function getCachedThumbnail(storedName: string): Promise<Buffer | null> {
+  try {
+    return await readFile(thumbPath(storedName));
+  } catch {
+    return null;
+  }
+}
+
+export async function saveThumbnail(storedName: string, buffer: Buffer): Promise<void> {
+  await ensureThumbDir();
+  await writeFile(thumbPath(storedName), buffer);
+}
+
+export async function deleteThumbnail(storedName: string): Promise<void> {
+  try {
+    await unlink(thumbPath(storedName));
+  } catch {
+    // Thumbnail may not exist
+  }
+}