nix: allow archive relay URL affinity override

evolution/proposals/ECP-0073-archive-relay-affinity-override.md

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+# ECP-0073: Archive Relay Affinity Override
+
+## Context
+
+`wt-archive` workers discover streams from `/api/public-streams` and subscribe to the listed `relay_url`. In practice, `cdn.moq.dev` resolves to region-local relay IPs, and broadcasts published from one region are not consistently visible from another region endpoint.
+
+For archive nodes running in a different region than publishers, this causes workers to connect successfully but wait indefinitely for broadcast announcements.
+
+## Decision
+
+Add an explicit Nix module option:
+
+- `services.every-channel.ec-node.archive.relayUrlOverride` (nullable string)
+
+When set, archive workers ignore per-entry `relay_url` from directory listings and always subscribe through the configured override URL.
+
+This allows operators to pin archive ingestion to the same relay endpoint used by publishers without changing public directory payloads.
+
+## Why
+
+- Restores deterministic archival ingestion across regions.
+- Keeps deployment-level control in Nix (no app-level migration needed).
+- Reversible with a single config change.
+
+## Rollout
+
+1. Set `archive.relayUrlOverride` on archive hosts that need relay affinity.
+2. If override uses an IP literal, enable `archive.tlsDisableVerify = true` until SNI-preserving IP overrides are implemented.
+3. Rebuild host and verify manifest growth.
+
+## Reversibility
+
+Unset `archive.relayUrlOverride` to return to directory-provided `relay_url` behavior.