Add HTTP gateway with webhook ingestion and bearer auth

[bug-ops]

Epic: #367

Description

Axum-based HTTP server for webhook ingestion from external services (Discord, Slack, WhatsApp). Binds localhost by default, requires bearer token auth.

Tasks

• zeph-gateway module or extend existing axum setup from zeph-a2a
• POST /webhook endpoint accepting JSON payload with Authorization: Bearer <token> header
• GET /health endpoint returning 200 OK with component status
• Rate limiting (per-IP sliding window)
• Body size limit (1 MiB)
• Bind 127.0.0.1 by default, refuse 0.0.0.0 without explicit config
• Config: [gateway] section with bind, port, auth_token