feat(shard)!: declare wire protocol 3

The site's declared version is the admin-set uo_link_config.protocol column, so
the sidecar's PROTOCOL_VERSION 2 -> 3 bump has to be matched here or every REST
call 409s and uoLinkSocket closes the WS on the ws.hello mismatch. Five places
carry the number and all five move together: the column default, the model's
DEFAULT_PROTOCOL (what a site with nothing saved yet declares), the two
`config.protocol || 1` fallbacks in uoLinkClient/uoLinkSocket -- unreachable
today, but an unset value quietly sending 1 is exactly the confusing 409 the
version check exists to prevent -- the admin form's initial value, and the
documented env default.

The boot migration is the only subtle part. schema.sql is re-run on EVERY boot,
and `protocol` is admin-editable, so a bare UPDATE would silently un-pin an
operator who had deliberately pinned an older sidecar in Admin -> Shard. It is
therefore gated on a marker row in `settings`, written after the UPDATE: the
first boot on this build migrates, every later boot is a no-op. `protocol < 3`
rather than `= 2` picks up an install still on the old default of 1, which could
not have been talking to a v2 sidecar anyway. A fresh install has no row to
update and just gets the marker plus the new column default.

Verified against the local MariaDB through ensureSchema (the production path):
2 -> 3 with the marker written and the column default now 3; pinned back to 2 by
hand, re-ran, and it STAYED 2 -- the one-shot property holds. 673 server tests,
47 client tests, client build green.

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

.env.example

@@ -117,7 +117,10 @@ BOT_INTERNAL_KEY=change-me-to-a-long-random-string
 # token). These URLs are just defaults; the admin can override them at runtime.
 UOLINK_BASE_URL=http://127.0.0.1:8080
 UOLINK_WS_URL=ws://127.0.0.1:8080/ws
-UOLINK_PROTOCOL=1
+# Wire protocol this build speaks (3 = Protocol 3.0). Only a fallback for a site
+# with nothing saved yet — the admin panel's pinned value wins — but set it lower
+# if you deliberately run an older sidecar.
+UOLINK_PROTOCOL=3
 
 # ─── Push notifications (M7) — self-hosted ntfy UnifiedPush relay ───
 # The `ntfy` compose service and the backend's push fan-out (opt-in notifications