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.

internal/config/config_test.go

@@ -35,6 +35,37 @@ func TestSecretPrecedence(t *testing.T) {
 	}
 }
 
+// Review M-12: fail fast on unusable configuration at startup.
+func TestValidate(t *testing.T) {
+	if err := (&Config{}).Validate(); err == nil {
+		t.Fatal("missing secret passed validation")
+	}
+	if err := (&Config{AuthSecret: "secret", NextAuthURL: "not a url"}).Validate(); err == nil {
+		t.Fatal("relative NEXTAUTH_URL passed validation")
+	}
+	for _, u := range []string{"", "http://localhost:3000", "https://app.example.com"} {
+		if err := (&Config{AuthSecret: "secret", NextAuthURL: u}).Validate(); err != nil {
+			t.Fatalf("valid configuration %q rejected: %v", u, err)
+		}
+	}
+}
+
+// Review M-12: the parsed ADMIN_EMAILS list is memoized but still follows
+// env changes (tests and operator restarts rely on that).
+func TestAdminEmailsTracksEnv(t *testing.T) {
+	t.Setenv("ADMIN_EMAILS", " A@Example.com ,, b@example.com ")
+	if got := AdminEmails(); len(got) != 2 || got[0] != "a@example.com" || got[1] != "b@example.com" {
+		t.Fatalf("unexpected parse: %v", got)
+	}
+	t.Setenv("ADMIN_EMAILS", "")
+	if got := AdminEmails(); len(got) != 0 {
+		t.Fatalf("cache did not follow the env change: %v", got)
+	}
+	if IsAdminEmail("a@example.com") {
+		t.Fatal("stale admin email after env change")
+	}
+}
+
 // Review OPS-01: a dedicated ENCRYPTION_KEY decouples integration-credential
 // encryption from the session signing secret.
 func TestEncryptionSecretPrecedence(t *testing.T) {