archive replay: add HLS DVR serve path and web mode

evolution/proposals/ECP-0071-archive-replay-dvr.md

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+# ECP-0071: Archive Replay DVR Endpoints
+
+## Context
+
+ECP-0070 added relay archival (`wt-archive`) into CAS objects plus JSONL indexes, but there is no read path for viewers to scrub historical content.
+
+## Decision
+
+Add an archive replay path with these pieces:
+
+- `ec-node wt-archive-serve`: lightweight HTTP server that reads archive JSONL indexes + CAS blobs and serves:
+  - `/archive/<broadcast>/master.m3u8`
+  - `/archive/<broadcast>/video.m3u8`
+  - `/archive/<broadcast>/audio.m3u8`
+  - `/archive/<broadcast>/init.mp4`
+  - `/archive/<broadcast>/segment/<blake3>.m4s`
+  - `/archive/<broadcast>/timeline.json`
+- HLS playlists are `EVENT` style and include all retained segments by default, so scrub depth is bounded by retained archive data (effectively "infinite" until eviction).
+- Cloudflare Worker proxies `/api/archive/*` to a configurable archive origin (`EC_ARCHIVE_ORIGIN`) so `every.channel` can serve replay from the same site domain.
+- Web watcher adds an `Archive DVR` mode that plays `/api/archive/<broadcast>/master.m3u8` with native HLS or `hls.js` fallback.
+- NixOS module adds `services.every-channel.ec-node.archive.serve.*` to run replay server alongside archival workers.
+
+## Rationale
+
+- Keeps archival and replay in one binary and one deploy surface.
+- Preserves CAS as source of truth; playlists are derived views.
+- Uses standard HLS+DVR semantics so browser playback + scrubbing works without custom protocol work in the short term.
+
+## Reversibility
+
+- Disable `archive.serve.enable` and remove worker proxy route to revert to archive-only mode.
+- `wt-archive` storage format remains unchanged.