fix(file-watcher): bound concurrent photo processing (#846)

* fix(file-watcher): bound concurrent photo processing

chokidar fires 'add' once per file — with no ignoreInitial option the
boot scan fires it for every existing file, and a bulk drop into the
watch folder fires it for every new one at once. Each handler runs DB
lookups plus (for new files) a full sharp pipeline; sharp.concurrency(2)
only caps libvips threads WITHIN one operation, not the number of
parallel pipelines, so unbounded handlers can OOM small hosts.

Gate both 'add' and 'unlink' through a shared p-limit
(FILE_WATCHER_CONCURRENCY, default 2, floor 1) — mass deletes otherwise
burst DB work and ZIP-cache invalidation the same way. p-limit is pinned
to ^3.1.0, the last CommonJS release.

Adapted from the filpgame fork (426ca491) — thanks @filpgame; extended
to cover 'unlink', documented in .env.example, plus a lock-in test for
the existing Sharp cache/concurrency caps this bound relies on.

* chore(compose): pass FILE_WATCHER_CONCURRENCY into the backend container (codex review of #846)

The backend service uses an explicit environment list (no env_file), so
the documented override never reached the container in the default
compose deployments. Added to both compose files + root .env.example.

backend/__tests__/services/imageProcessor.sharpConfig.test.js

@@ -0,0 +1,30 @@
+/**
+ * Locks the process-wide Sharp memory guards. The file-watcher concurrency
+ * bound (FILE_WATCHER_CONCURRENCY) assumes these caps stay in place — they
+ * limit libvips threads/cache WITHIN one operation while p-limit bounds the
+ * number of parallel pipelines. From the filpgame fork (426ca491).
+ */
+
+const mockSharp = jest.fn();
+mockSharp.cache = jest.fn();
+mockSharp.concurrency = jest.fn();
+
+jest.mock('sharp', () => mockSharp);
+
+jest.mock('../../src/utils/logger', () => ({
+  error: jest.fn(),
+  warn: jest.fn(),
+  info: jest.fn(),
+  debug: jest.fn(),
+}));
+
+describe('imageProcessor Sharp configuration', () => {
+  it('disables the Sharp cache and caps libvips concurrency', () => {
+    jest.isolateModules(() => {
+      require('../../src/services/imageProcessor');
+    });
+
+    expect(mockSharp.cache).toHaveBeenCalledWith(false);
+    expect(mockSharp.concurrency).toHaveBeenCalledWith(2);
+  });
+});