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/lib/reference-data.ts

@@ -19,6 +19,13 @@ export function invalidateReference(kind: ReferenceKind) {
   cache.delete(kind)
 }
 
+// Review L-6: the cache is module-global, not scoped to the authenticated
+// identity. An in-tab identity change (sign-out, view-as enter/exit) must not
+// serve the previous user's me/tags/contacts for up to 30 seconds.
+export function resetReferenceCache() {
+  cache.clear()
+}
+
 // Resolves to null on any failure; failures are not kept cached.
 export function fetchReference<T = unknown>(kind: ReferenceKind): Promise<T | null> {
   const hit = cache.get(kind)