feat: Add gateway and DNS fields to pool model

[mgazza]

Problem

Pool definitions only store CIDR, prefix, exclusions, and metadata. Gateway and DNS configuration is missing, forcing each BNG node to be independently configured with --pool-gateway and --pool-dns CLI flags.

In a distributed deployment with multiple BNG nodes sharing a Nexus pool, this means:

• No single source of truth for gateway/DNS per pool
• Misconfiguration risk (different BNGs could serve different gateways for the same pool)
• Breaks the ZTP story — BNG can auto-discover Nexus but still needs manual gateway/DNS config

Raised by @infinitydon in codelaboratoryltd/bng#92.

Proposed Changes

Add gateway and dns fields to the pool model and API:

Pool creation request:

{
  "id": "default",
  "cidr": "10.0.1.0/24",
  "prefix": 32,
  "gateway": "10.0.1.1",
  "dns": ["8.8.8.8", "8.8.4.4"]
}

Files to modify:

• internal/store/models.go — Add Gateway and DNS fields to Pool struct
• internal/api/pools.go — Add fields to PoolRequest and PoolResponse
• internal/validation/ — Add validation for gateway (must be valid IP within or adjacent to CIDR) and DNS (valid IP addresses)

Acceptance Criteria

• POST /api/v1/pools accepts optional gateway (string IP) and dns (array of string IPs)
• GET /api/v1/pools/{id} returns gateway and dns in response
• Gateway validated as valid IPv4/IPv6 address
• DNS entries validated as valid IP addresses
• Fields are optional (backwards compatible — existing pools without gateway/dns continue to work)
• Tests updated

[mgazza]

Fixed in PR #35 — gateway and DNS fields added to pool model