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/signup/client.tsx

@@ -18,7 +18,9 @@ export function SignupClient() {
 
   const handleSignup = async (e: React.FormEvent) => {
     e.preventDefault()
-    if (password.length < 6) { toast.error('Password must be at least 6 characters'); return }
+    // Review M-8: match the server's 8-character minimum (and the input's
+    // minLength). The old 6-character check let a doomed request through.
+    if (password.length < 8) { toast.error('Password must be at least 8 characters'); return }
     setLoading(true)
     try {
       const res = await fetch('/api/signup', {