Address code review findings across API, web and deployment

High severity:
- wire TRUSTED_PROXY through Compose/deploy/docs and warn at web startup
- bucket public rate limits on effective client IP, add per-bookmark budget,
  and forward the signed browser identity from the share page
- disable first-run bootstrap in server deployments and document seeding
- serialize last-admin demotion in one advisory-locked transaction

Medium/low:
- cap and batch share recipient inserts
- add a shared outbound budget for preview/CardDAV/IMAP/signal work
- pin CardDAV dials to 443 and bound IMAP logout
- fix rightward kanban column drags and normalize cards on column delete
- classify backend outages in auth guards, add request timeouts and an
  error boundary, remove the dead login route, surface throttling
- enforce pinned/unpinned page reorder grouping server-side
- avoid preview work for stale bookmark versions; drop duplicate dialog fetch
- sequence-guard collection/tag/calendar/quick-find loads
- validate property config and fix MULTI_SELECT/DATE filter inputs
- document index maintenance; apply same-origin check to sign-out
- trim signup names, unique shared dates and kanban keys (migration 010),
  reset reference cache on identity changes, hide OIDC issuer, return 413
  for oversized direct bodies, escape Markdown link destinations
- remove dead template files and duplicate test helper

Verified with go build/vet, full PostgreSQL race suite, db convergence,
tsc, both ESLint configs and a Next production build.

web/app/(app)/error.tsx

@@ -0,0 +1,28 @@
+'use client'
+
+// Review M-6: a backend outage is not a logout. Server components throw
+// BackendUnavailableError (auth guards) or fail their data fetch; show a
+// retry surface instead of redirecting to /login or rendering empty data.
+import { useEffect } from 'react'
+
+export default function AppError({ error, reset }: { error: Error & { digest?: string }; reset: () => void }) {
+  useEffect(() => {
+    console.error('App route failed', error)
+  }, [error])
+
+  return (
+    <main className="mx-auto flex min-h-[60vh] max-w-md flex-col items-center justify-center gap-4 p-6 text-center">
+      <h1 className="text-xl font-semibold">Service temporarily unavailable</h1>
+      <p className="text-sm text-muted-foreground">
+        The application backend could not be reached. You are still signed in; try again in a moment.
+      </p>
+      <button
+        type="button"
+        onClick={() => reset()}
+        className="rounded-md bg-primary px-4 py-2 text-sm font-medium text-primary-foreground hover:opacity-90"
+      >
+        Try again
+      </button>
+    </main>
+  )
+}