feat: cloud adapters E2B/Modal and billing export (SAND-WP-0010)

Add credentialed E2B and Modal extensions, burst routing fallback,
fin-hub meter export hook, BYOK docs, and 77 tests.

src/sandboxer/payments/billing_export.py

@@ -0,0 +1,49 @@
+"""Optional fin-hub billing export for metered sandbox usage."""
+
+from __future__ import annotations
+
+import os
+from typing import Any
+
+import httpx
+
+from sandboxer.models import MeterRecord, SandboxStatus
+
+
+def fin_hub_url() -> str | None:
+    return os.environ.get("SANDBOXER_FIN_HUB_URL") or None
+
+
+def export_meter_usage(
+    status: SandboxStatus,
+    *,
+    extension_id: str,
+    meter: MeterRecord,
+) -> dict[str, Any] | None:
+    """POST usage record to fin-hub when SANDBOXER_FIN_HUB_URL is set."""
+    if os.environ.get("SANDBOXER_NO_FIN_HUB", "").lower() in ("1", "true", "yes"):
+        return None
+    if meter.pricing_model != "metered" or not meter.actual_usd:
+        return None
+
+    base = fin_hub_url()
+    if not base:
+        return None
+
+    payload = {
+        "sandbox_id": status.sandbox_id,
+        "extension_id": extension_id,
+        "profile_id": status.profile_id,
+        "consumer": status.consumer.model_dump(),
+        "duration_s": meter.duration_s,
+        "actual_usd": meter.actual_usd,
+        "estimate_usd": meter.estimate_usd,
+        "currency": meter.currency,
+    }
+
+    try:
+        response = httpx.post(f"{base.rstrip('/')}/usage/sandbox", json=payload, timeout=10.0)
+        response.raise_for_status()
+        return response.json()
+    except httpx.HTTPError:
+        return None