ProdCom 1.3.0: one connection to ProdCom, shared by every dashboard

The module used to proxy each browser's transcript stream to ProdCom one
to one, so N screens meant N upstream connections and N history reads.
It now follows ProdCom once: it keeps the channel list, the groups and
the last 300 entries, and gives every browser a snapshot on connect
(`state`) plus each changed entry as it happens (`entry`), with `clear`
and `status` frames as the link changes. Reconnects back off from 2 s to
15 s, re-read the lists and history, and re-send the snapshot. The tile
no longer loads anything itself; the admin readout says how many channels
and entries the server holds. The /prodcom/* proxy is gone.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

README.md

@@ -15,9 +15,10 @@ touching the shell (see [docs/MODULE-GUIDE.md](docs/MODULE-GUIDE.md)).
 2. Discovers modules in `modules/` and serves each one's client at
    `/modules/<id>/`.
 3. Mounts each enabled module's server routes under `/api/modules/<id>/` —
-   that's where module servers proxy ProdCom, poll ProPresenter, and stream
-   live updates over SSE, so browsers never talk to production gear
-   directly (no CORS, and API keys never leave the server).
+   that's where module servers follow ProdCom, poll ProPresenter, and stream
+   live updates over SSE — one upstream connection per module, however many
+   dashboards are open — so browsers never talk to production gear directly
+   (no CORS, and API keys never leave the server).
 4. Stores server-wide module config and named layouts in a per-machine data
    directory outside the app folder, so updating ProdDash never loses them.