feat(billing): extract Billing Service (Step 4)

- stripe_service: checkout sessions, webhook handling, subscription CRUD
- tier_manager: feature matrix (4 tiers), quota enforcement, rate limits
- routes: checkout, webhook (no auth), subscription, tier query, features
- Traefik header auth (X-User-Id) replaces get_current_user dependency
- /tier/{user_id} endpoint for internal service-to-service lookups
- /features and /features/{tier} for feature matrix queries
- Dockerfile: single worker, 30s timeout (lightweight service)

services/billing/Dockerfile

@@ -0,0 +1,36 @@
+# ── builder ──────────────────────────────────────────────────────────────────
+FROM python:3.12-slim AS builder
+
+WORKDIR /build
+
+COPY services/billing/requirements.txt ./requirements.txt
+RUN pip install --upgrade pip && \
+    pip install --no-cache-dir --prefix=/install -r requirements.txt
+
+# ── runtime ──────────────────────────────────────────────────────────────────
+FROM python:3.12-slim AS runtime
+
+RUN addgroup --system appgroup && adduser --system --ingroup appgroup appuser
+
+WORKDIR /app
+
+COPY --from=builder /install /usr/local
+
+# Shared module
+COPY shared/ shared/
+
+# Service source
+COPY services/billing/app/ app/
+
+RUN chown -R appuser:appgroup /app
+
+USER appuser
+
+EXPOSE 8000
+
+# Billing is lightweight — single worker is fine
+CMD ["gunicorn", "app.main:app", \
+     "-k", "uvicorn.workers.UvicornWorker", \
+     "--bind", "0.0.0.0:8000", \
+     "--workers", "1", \
+     "--timeout", "30"]