docs: the shard screen's real path, and the two keys a deploy cannot boot without

Two operator-facing errors, both found by following the documentation exactly
against a real deployment while writing the runicgateway.com installation
journey.

**The shard screen moved and the guide did not.** INSTALL.md §5 and PLAN.md both
print the installer's handoff block, which said to paste the four values at
`<site>/admin/shard`. Those screens belong to the `uo` module now, and a module
owns one path segment wherever it appears (MODULE_SYSTEM.md §2.8), so the page
is `/admin/uo/link`. The old path does not 404 — the SPA sends the operator to
the dashboard, so the link looks like it worked and the values have nowhere to
go. The instruction under the block now names the path, and a note warns anyone
whose earlier run printed the old one. API routes are unaffected by the module
rule and keep `/api/v1/admin/shard/*`; the note says so, because that is the
distinction the next editor will need. Matches installer#22.

**The README snapshot's quickstart could not boot.** website's root
`.env.example` never listed SECRET_ENC_KEY, and the "set at least" list omitted
both it and BOT_INTERNAL_KEY — each of which the server refuses to start without
in production, BOT_INTERNAL_KEY even on a deployment running no bot. Synced from
website#163, which fixes the file itself and was verified by booting the
published image from a clean `cp .env.example .env`.

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

installer/PLAN.md

@@ -1068,8 +1068,8 @@ One manual step remains — connect the website to this sidecar:
   Protocol version  3
   Auth token        4f9c...   (also in /etc/runicgateway/sidecar.toml)
 
-Paste these into  Admin → Shard  on your Runic Gateway site:
-  https://<your-site>/admin/shard
+Paste these into  Admin → Shard (uo-link)  on your Runic Gateway site:
+  https://<your-site>/admin/uo/link
 
 The token is write-only once saved — the site will never show it back to you.
 ```