Add gateway nodes for public workload ingress

[flo2517]

Introduce gateway nodes that terminate public ingress and route traffic into private workloads on the mesh, so that a workload can be reachable from the internet without any provider exposing its own network directly. Gateways are a new role with a new trust boundary: per ADR-012 §2 they are trusted for reachability only and must never be trusted with the confidentiality or integrity of tenant traffic.

Blocked by: ADR-017 (see ADR-012 §6) — no implementation before acceptance. Depends on #53.

Acceptance criteria:

• gateway registration, discovery, and selection are decentralized and do not reintroduce a single operator on the ingress path;
• tenant traffic is end-to-end protected through the gateway; a malicious gateway can drop or delay traffic but cannot read or alter it undetected;
• routing to a workload is authorized by a finalized lease, and revocation removes reachability within a stated bound;
• gateways are rate-limited and abuse-resistant, and a gateway cannot be used to amplify traffic at a third party;
• gateway operators are accountable: their availability is measurable and attributable, so the reputation and penalty machinery can apply to them as it does to providers;
• selective blackholing and metadata harvesting are addressed explicitly, per the gateway row of the ADR-012 §2 threat table;
• degraded state — no gateway available for a workload — is reported as an explicit error, never as a healthy deployment;
• tests cover ingress to a healthy workload, gateway failure and failover, lease revocation, a gateway attempting interception, and abusive traffic from the public side.

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Original request (FR, preserved verbatim):

Créer des nodes spécialisés pour router le trafic public vers les workloads privés du mesh.