feat(maturity): add dedicated NSFW/maturity analysis service

- Add maturity/ service (FastAPI + Falconsai/nsfw_image_detection ViT classifier)
  - /analyze (URL) and /analyze/file (multipart upload) endpoints
  - Normalized response: maturity_label, confidence, score, labels,
    action_hint (safe/review/flag_high), advisory, threshold_used,
    analysis_time_ms, model, source
  - Configurable thresholds via MATURITY_THRESHOLD_MATURE / MATURITY_THRESHOLD_REVIEW
  - Reuses common/image_io for URL validation and file-size enforcement
  - Explicit 502/503 errors on failure — no silent safe fallback
  - Per-request structured logging (score, label, threshold path, elapsed ms)

- Update gateway/main.py
  - MATURITY_URL + MATURITY_ENABLED env vars
  - POST /analyze/maturity and POST /analyze/maturity/file endpoints
  - /health includes maturity service status
  - _assert_maturity_enabled() guard for clean 503 when disabled
  - All existing endpoints untouched (additive change)

- Update docker-compose.yml
  - Add maturity service with healthcheck (start_period: 90s)
  - Gateway environment: MATURITY_URL, MATURITY_ENABLED
  - Gateway depends_on: maturity (service_healthy)

- Update README.md and USAGE.md
  - Document maturity service, env vars, curl examples,
    full response schema table, action_hint logic, failure guidance

maturity/Dockerfile

@@ -0,0 +1,17 @@
+FROM python:3.11-slim
+
+WORKDIR /app
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    ca-certificates \
+  && rm -rf /var/lib/apt/lists/*
+
+COPY maturity/requirements.txt /app/requirements.txt
+RUN pip install --no-cache-dir -r /app/requirements.txt
+
+COPY maturity /app
+COPY common /app/common
+
+ENV PYTHONUNBUFFERED=1
+
+CMD ["uvicorn", "main:app", "--host", "0.0.0.0", "--port", "8000"]