feat: pre-generate Directus image previews

Bundle extension (upload hook + API endpoint + admin module) that warms
Directus image transform cache in-process with an explicit acceptFormat,
so the generated file matches the browser's cache key byte-for-byte.

- resumable, window-aware backfill with progress + cancel
- system + custom presets, effective-format reporting, shadow warnings
- storage breakdown (previews vs originals), human-readable API fields
- diagnostics: resolve-plan prediction, per-file generate, ops ring buffer
- isolated dev/staging compose stack + deploy/rollback scripts

Verified on Directus 11.17.4: 40/40 tests, hash-match gate passed,
idempotent backfill, custom fixed-format presets, error surfacing.

dev/compose.dev.yml

@@ -0,0 +1,51 @@
+name: image-previews-dev
+
+services:
+  db:
+    image: postgres:16-alpine
+    environment:
+      POSTGRES_USER: directus
+      POSTGRES_PASSWORD: directus
+      POSTGRES_DB: directus
+    volumes:
+      - db-data:/var/lib/postgresql/data
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U directus"]
+      interval: 5s
+      timeout: 5s
+      retries: 20
+
+  directus:
+    image: directus/directus:11.17.4
+    ports:
+      - "8057:8055"
+    depends_on:
+      db:
+        condition: service_healthy
+    environment:
+      KEY: dev-key
+      SECRET: dev-secret
+      ADMIN_EMAIL: admin@example.com
+      ADMIN_PASSWORD: admin12345
+      DB_CLIENT: pg
+      DB_HOST: db
+      DB_PORT: "5432"
+      DB_DATABASE: directus
+      DB_USER: directus
+      DB_PASSWORD: directus
+      STORAGE_LOCATIONS: local
+      STORAGE_LOCAL_ROOT: /directus/uploads
+      EXTENSIONS_AUTO_RELOAD: "true"
+      CACHE_ENABLED: "false"
+      RATE_LIMITER_ENABLED: "false"
+      ASSETS_TRANSFORM_MAX_CONCURRENT: "2"
+      IMAGE_PREVIEWS_ENABLED: "true"
+      IMAGE_PREVIEWS_CONCURRENCY: "1"
+      LOG_LEVEL: info
+    volumes:
+      - uploads:/directus/uploads
+      - ./extension:/directus/extensions/directus-extension-generate-image-previews
+
+volumes:
+  db-data:
+  uploads: