feat(webhooks): add lifecycle-event webhooks support - #4

Summary

Adds support for the E2B lifecycle-event webhooks API — both halves:

• Outbound management (E2bEx.Webhooks) — CRUD over /events/webhooks (list/1, create/2, get/2, update/3, delete/2), mirroring the existing E2bEx.Volumes resource and funnelling through the single E2bEx.Request HTTP chokepoint. Decoded into the new E2bEx.Webhook struct.
• Inbound delivery (E2bEx.WebhookEvent) — a pure, HTTP-free module that decodes the delivered (snake_case) event payload and verifies its signature:
  • verify_signature/3 — plain SHA256 of secret <> raw_body, standard base64, trailing = stripped, constant-time compared to the e2b-signature header.
  • parse/3 — verify-then-decode → {:ok, %WebhookEvent{}} | {:error, :invalid_signature | :invalid_payload}.

Notes

• These endpoints are not in openapi.yml (absent from both this repo's copy and upstream E2B's), and the JS/Python SDKs don't implement webhook management either. Source of truth is the E2B docs. Recorded as a gotcha in CLAUDE.md.
• Deliberate divergences (documented in CLAUDE.md): the inbound payload is already snake_case so WebhookEvent.from_api/1 reads keys directly (no camelCase conversion); and parse/3 returns atom error reasons rather than %E2bEx.Error{}, since signature/JSON validation is local, not an HTTP failure (mirrors how Commands diverges).
• create/2/update/3 return {:error, %E2bEx.Error{reason: :empty_response_body}} instead of crashing if the API returns an empty body (the response shape for these is unconfirmed).

Process

Built via the project's superpowers workflow: spec → plan → subagent-driven TDD, with per-task spec+quality reviews and a final whole-branch review.

Test plan

• mix test — 150 tests, 0 failures.
• mix compile --warnings-as-errors — clean.
• New coverage: CRUD (method/path/body/decode + non-2xx error + empty-body error), WebhookEvent.from_api/1 decoding, and signature/parse cases (valid, tampered body, wrong secret, length mismatch, non-JSON-with-valid-sig).

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