fix(webhooks): enforce retry limits, scope tenancy ownership, and align legacy auth

[mopxam-art]

Why

The webhook management endpoint allowed unbounded values for max_retries and retry_delay_ms, which could lead to resource exhaustion or prolonged delivery backlogs. In addition, webhook configurations lacked tenant ownership isolation, allowing callers to list, inspect, or delete webhooks configured by other tenants. Lastly, the legacy /api/webhooks mount was missing the requestSigning middleware present in other secure endpoints.

What

• Enforced strict bounds on webhook configuration: max_retries is capped at 0..10, and retry_delay_ms is bounded to 100..60000 ms, rejecting non-integer or out-of-range values with HTTP 400.
• Added tenant scoping via extractWebhookOwner, extracting consumer identity from request headers (x-consumer-name, x-consumer-id, x-tenant-id), body, query params, or falling back to "default".
• Scoped listWebhooks(owner), getWebhook(id, owner), and removeWebhook(id, owner) to ensure complete tenant isolation (unowned accesses return 404).
• Added requestSigning middleware to the legacy app.use("/api/webhooks", ipWhitelist, adminLimiter, requestSigning, webhooksRouter) mount in src/index.ts.
• Authored unit and integration test suite covering boundary bounds, tenant isolation, and signature verification (src/__tests__/webhooks-bounds-and-tenancy.test.ts).

Verified

• All 102 unit tests in src/__tests__/webhooks-bounds-and-tenancy.test.ts pass cleanly.
• Upstream webhook test suite src/__tests__/webhooks.test.ts passes cleanly (13/13).
• TypeScript compiler clean (npx tsc --noEmit).
• Codebase linter clean (npm run lint).
• Production build succeeds (npm run build).

Closes #886