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added relay archival (wt-archive) into CAS objects plus JSONL indexes, but there is no read path for viewers to scrub historical content.

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.

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.

Disable archive.serve.enable and remove worker proxy route to revert to archive-only mode.
wt-archive storage format remains unchanged.