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/README.md

@@ -0,0 +1,48 @@
+# Local dev / staging stack
+
+Isolated Directus **11.17.4** with its own Postgres and volumes. Nothing is shared with the prod
+instance (separate containers, volumes, and port **8057**).
+
+## Run
+
+```bash
+# from the project root
+./dev/sync-extension.sh          # build + copy dist/package.json into dev/extension
+docker compose -f dev/compose.dev.yml up -d
+```
+
+Admin: `http://localhost:8057` — `admin@example.com` / `admin12345`.
+The extension is loaded from `dev/extension` (bind mount). Check:
+
+```bash
+docker logs image-previews-dev-directus-1 | grep -i "loaded extensions"
+```
+
+## After changing source
+
+```bash
+./dev/sync-extension.sh
+docker compose -f dev/compose.dev.yml restart directus
+```
+
+(`EXTENSIONS_AUTO_RELOAD` is on, but a restart is the reliable path. Never `rm -rf dev/extension` —
+it breaks the bind mount.)
+
+## Smoke test
+
+```bash
+BASE=http://localhost:8057
+TOKEN=$(curl -s -X POST "$BASE/auth/login" -H 'Content-Type: application/json' \
+  -d '{"email":"admin@example.com","password":"admin12345"}' | jq -r '.data.access_token')
+
+curl -s "$BASE/image-previews/status" -H "Authorization: Bearer $TOKEN" | jq .
+curl -s -X POST "$BASE/files" -H "Authorization: Bearer $TOKEN" -F "file=@dev/seed/test.jpg" | jq '.data | {id,filename_disk}'
+docker exec image-previews-dev-directus-1 sh -c "ls -1 /directus/uploads | grep '__' | sort"
+```
+
+## Stop / reset
+
+```bash
+docker compose -f dev/compose.dev.yml down          # stop, keep data
+docker compose -f dev/compose.dev.yml down -v       # stop and wipe DB + uploads
+```