feat: add v2 stack (backend, runner, ui-v2) with release workflow

- backend/: Go API server and runner binaries with PocketBase + MinIO storage
- ui-v2/: SvelteKit frontend rewrite
- docker-compose-new.yml: compose file for the v2 stack
- .gitea/workflows/release-v2.yaml: CI/CD for backend, runner, and ui-v2 Docker Hub images
- scripts/pb-init.sh: migrate from wget to curl, add superuser bootstrap for fresh installs
- .env.example: document DOCKER_BUILDKIT=1 for Colima users

.env.example

@@ -1,6 +1,23 @@
 # libnovel scraper — environment overrides
 # Copy to .env and adjust values; do NOT commit this file with real secrets.
 
+# ── Docker BuildKit ───────────────────────────────────────────────────────────
+# Required for the backend/Dockerfile cache mounts (--mount=type=cache).
+# BuildKit is the default in Docker Engine 23+, but Colima users may need this.
+#
+# If you see: "the --mount option requires BuildKit", enable it one of two ways:
+#
+#   Option A — per-project (recommended, zero restart needed):
+#     Uncomment the line below and copy this file to .env.
+#     Docker Compose reads .env automatically, so BuildKit will be active for
+#     every `docker compose build` / `docker compose up --build` in this project.
+#
+#   Option B — system-wide for Colima (persists across restarts):
+#     echo '{"features":{"buildkit":true}}' > ~/.colima/default/daemon.json
+#     colima stop && colima start
+#
+# DOCKER_BUILDKIT=1
+
 # ── Service ports (host-side) ─────────────────────────────────────────────────
 # Port the scraper HTTP API listens on (default 8080)
 SCRAPER_PORT=8080