ec-node: add relay CAS archiver and nix auto-archive service

evolution/proposals/ECP-0070-relay-cas-archival.md

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+# ECP-0070: Relay-Native CAS Archival + NixOS Auto-Archive Service
+
+## Summary
+
+Add a first-party archival path for MoQ relay streams:
+
+1. `ec-node wt-archive`:
+- Subscribe to a relay broadcast over WebTransport/MoQ.
+- Persist each received group payload as a content-addressed object (BLAKE3).
+- Append per-track JSONL index records ("manifests") that reference CAS object hashes/paths.
+
+2. Extend the NixOS `services.every-channel.ec-node` module with `archive.*` options:
+- Poll `every.channel` public stream directory.
+- Auto-start one `wt-archive` worker per discovered broadcast.
+- Store large CAS blobs and manifest indices in separately configurable roots.
+
+## Motivation
+
+We need durable, low-duplication TV archive storage on dedicated storage hosts.
+
+- Relay ingestion makes archive nodes location-agnostic.
+- CAS enables deduplication and integrity-friendly storage semantics.
+- Separate manifest/index storage allows faster metadata operations and simpler backup policy.
+- Nix-managed host config keeps archival behavior immutable and repeatable.
+
+## Decision
+
+- Introduce `ec-node wt-archive` as the minimal relay subscriber for archival.
+- Use BLAKE3 object IDs and a sharded filesystem layout (`<cas-root>/objects/blake3/aa/<hash>.bin`).
+- Write append-only JSONL records per `broadcast/track` under a configurable manifest directory.
+- Add `services.every-channel.ec-node.archive.*` in the Nix module to auto-discover and archive public streams.
+
+Initial default archived tracks target current relay publisher output:
+- `catalog.json`
+- `init.mp4`
+- `video0.m4s`
+- `audio0.m4s`
+
+## Consequences
+
+Pros:
+- Immediate archival path without requiring browser components.
+- Works with current Cloudflare relay + WebTransport deployment.
+- Fully declarative deployment via existing Nix input wiring.
+
+Tradeoffs:
+- First version archives raw group payloads and does not yet normalize across track schema variants.
+- Discovery source is the web public stream list (not full control-topic gossip ingestion).
+- Per-broadcast workers are process-based and best-effort supervised.
+
+## Rollout
+
+1. Ship `wt-archive` command in `ec-node`.
+2. Extend Nix module with `archive.*`.
+3. Enable on storage host(s) with:
+- CAS root on large ZFS pool.
+- manifest root on separate filesystem/dataset.
+
+## Reversibility
+
+- Disable `services.every-channel.ec-node.archive.enable`.
+- Existing CAS/manifests remain inert on disk and can be pruned independently.