Integrate the riverui in area51 (#3745)

<img width="2618" height="1061" alt="image"
src="https://github.com/user-attachments/assets/9f2aa352-9528-452f-8e28-f776535dc4fe"
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<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Medium Risk**
> Adds a new mounted admin-capable UI (`riverui`) that starts background
processes and exposes job control actions, so misconfiguration of auth
scopes, routing order, or context cancellation could impact operations
or shutdown behavior.
>
> **Overview**
> Integrates the official River UI into Area51 by creating and starting
a `riverui` handler during router construction, mounting it at
`/area51/api-server/riverui` with `admin:write` scope protection, and
adding a navigation link.
>
> Refactors the Area51 router plumbing by renaming
`NewAdminRouter*`/`adminApp` to `NewRouter*`/`area51App`, adding an
`area51` lifecycle context from `service.Server` (with explicit
cancellation on shutdown and on startup failures), and updating
handlers/templates/tests to use the new app/type names. Dependencies are
updated to include `riverui` and a `slog`→logrus adapter for River UI
logging.
>
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
e6fc872d5daa4b9bb283ea465e3a2f48fe9c9485. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

GitOrigin-RevId: 68f673d4701e7776c953bf56f4128b9d297ab0d7

Author: tphoney

go.mod

@@ -146,6 +146,7 @@ require (
 	github.com/riverqueue/river/rivertype v0.30.2
 	github.com/riverqueue/rivercontrib/otelriver v0.7.0
 	github.com/rs/cors v1.11.1
+	github.com/samber/slog-logrus/v2 v2.5.2
 	github.com/sashabaranov/go-openai v1.41.2
 	github.com/serpapi/serpapi-golang v0.0.0-20260126142127-0e41c7993cda
 	github.com/sirupsen/logrus v1.9.4
@@ -385,6 +386,8 @@ require (
 	github.com/rs/xid v1.5.0 // indirect
 	github.com/sagikazarmark/locafero v0.11.0 // indirect
 	github.com/saintfish/chardet v0.0.0-20230101081208-5e3ef4b5456d // indirect
+	github.com/samber/lo v1.47.0 // indirect
+	github.com/samber/slog-common v0.18.1 // indirect
 	github.com/segmentio/asm v1.2.0 // indirect
 	github.com/sergi/go-diff v1.3.1 // indirect
 	github.com/shirou/gopsutil/v3 v3.23.7 // indirect

go.sum

@@ -895,6 +895,12 @@ github.com/sagikazarmark/locafero v0.11.0 h1:1iurJgmM9G3PA/I+wWYIOw/5SyBtxapeHDc
 github.com/sagikazarmark/locafero v0.11.0/go.mod h1:nVIGvgyzw595SUSUE6tvCp3YYTeHs15MvlmU87WwIik=
 github.com/saintfish/chardet v0.0.0-20230101081208-5e3ef4b5456d h1:hrujxIzL1woJ7AwssoOcM/tq5JjjG2yYOc8odClEiXA=
 github.com/saintfish/chardet v0.0.0-20230101081208-5e3ef4b5456d/go.mod h1:uugorj2VCxiV1x+LzaIdVa9b4S4qGAcH6cbhh4qVxOU=
+github.com/samber/lo v1.47.0 h1:z7RynLwP5nbyRscyvcD043DWYoOcYRv3mV8lBeqOCLc=
+github.com/samber/lo v1.47.0/go.mod h1:RmDH9Ct32Qy3gduHQuKJ3gW1fMHAnE/fAzQuf6He5cU=
+github.com/samber/slog-common v0.18.1 h1:c0EipD/nVY9HG5shgm/XAs67mgpWDMF+MmtptdJNCkQ=
+github.com/samber/slog-common v0.18.1/go.mod h1:QNZiNGKakvrfbJ2YglQXLCZauzkI9xZBjOhWFKS3IKk=
+github.com/samber/slog-logrus/v2 v2.5.2 h1:pReWs5r4u/NbZokrJkFoFhURA5CnlUAgcMcPV1U1LuY=
+github.com/samber/slog-logrus/v2 v2.5.2/go.mod h1:wHDewbid6WQqF8E1HfMGeQtd+nAs9aqZgWCHoiGy4sQ=
 github.com/sashabaranov/go-openai v1.41.2 h1:vfPRBZNMpnqu8ELsclWcAvF19lDNgh1t6TVfFFOPiSM=
 github.com/sashabaranov/go-openai v1.41.2/go.mod h1:lj5b/K+zjTSFxVLijLSTDZuP7adOgerWeFyZLUhAKRg=
 github.com/segmentio/asm v1.2.0 h1:9BQrFxC+YOHJlTlHGkTrFWf59nbL3XnCoFLTwDCI7ys=