feat(auth): gate OSS signup behind ENABLE_SIGNUP flag (#514)

* feat(auth): gate OSS signup behind ENABLE_SIGNUP flag

## Problem

The `POST /api/v1/auth/signup` endpoint is unconditionally exposed on
every OSS install. Operators running an invite-only deployment (private
customer instances, staging environments, internal-only tenants) have
no way to disable public account creation without patching the codebase.
The UI also shows the "Sign up" link on `/auth/login` regardless of
whether signup is available, so a locked-down deployment leaves broken
navigation on the login page.

## Fix

Introduce a single `ENABLE_SIGNUP` env var (default `true` — no behavior
change for existing installs) that controls signup end-to-end:

- **Backend** — `api/constants.ENABLE_SIGNUP` is read at module load.
  The signup handler returns 403 when it's false. Also exposed on
  `GET /api/v1/health` as `signup_enabled: bool` so the UI can mirror
  the operator's choice at runtime instead of at bundle-build time.

- **UI** — `getSignupEnabled()` in `lib/auth/config.ts` proxies the
  health field, `/api/config/auth` surfaces it to the browser, the
  login page conditionally renders the "Sign up" link via a one-shot
  `fetch("/api/config/auth")` in `useEffect`, and the middleware
  redirects `/auth/signup` → `/auth/login` when disabled (fires before
  Next.js can serve the statically-prerendered signup page).

- **Helm** — `config.enableSignup` (default `true`) is rendered into
  the ConfigMap as `ENABLE_SIGNUP` so operators can flip it via
  `--set config.enableSignup=false` at install/upgrade time.

Fallbacks default to `signupEnabled: true` in every layer so a fresh
install "just works" and matches the backend default.

* address review: rollout on ConfigMap change, cache TTL, no signup-link flash

Four review points on #514:

**P1 — ConfigMap Change Skips Rollout** (`configmap.yaml`). `helm upgrade
--set config.enableSignup=false` updated the ConfigMap but did NOT roll
the api pods, so running processes kept the ENABLE_SIGNUP env from
startup and continued serving the old signup behavior — including
divergence between replicas mid-upgrade.

Fix: add the standard `checksum/config` pod-template annotation on the
four backend Deployments that `envFrom` the ConfigMap (`web`,
`arq-worker`, `ari-manager`, `campaign-orchestrator`). Verified with
`helm template`: all four Deployments share the same checksum on any
given render, and flipping `config.enableSignup` changes the checksum
uniformly so kubectl sees a pod-template diff and rolls all four.

**P1 — Signup Flag Stays Cached (server)** (`ui/src/lib/auth/config.ts`).
Module-scoped cache had no TTL. `revalidate: 300` was passed on the
underlying `fetch()` but the in-memory short-circuit above ran first, so
the value never refreshed until the UI pod restarted.

Fix: add `AUTH_CONFIG_TTL_MS = 5 * 60 * 1000` (matching the fetch
revalidate hint) so the module cache and the Next fetch cache stay in
sync. Backend flag flips propagate within 5 minutes without a pod
restart.

**P1 — Middleware Redirect Uses Stale State** (`ui/src/middleware.ts`).
Same shape as above — a separate module cache with no expiry could keep
redirecting `/auth/signup → /auth/login` after signup was re-enabled, or
keep serving the statically-prerendered signup page after lockdown.

Fix: same `SERVER_CONFIG_TTL_MS = 5 * 60 * 1000` TTL on the middleware
cache.

**P2 — Signup link flash on login page** (`ui/src/app/auth/login/page.tsx`).
Initial `signupEnabled` state was `null`, so `{signupEnabled && ...}`
hid the link on first paint and it popped in after the fetch resolved
— a CLS on every login-page load on stock installs where signup is
enabled.

Fix: initialise the state to `true` (matches the backend default). The
fetch still overrides to `false` when the operator has actually
disabled signup, so the lockdown UI behavior is unchanged; only the
happy-path flash is gone.

* simplify signup flag: drop TTL caches and middleware redirect

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* resolve signup flag server-side to avoid signup link flicker

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

---------

Co-authored-by: prabhat pankaj <prabhatiitbhu@gmail.com>
Co-authored-by: Abhishek Kumar <abhishek@a6k.me>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

docs/developer/environment-variables.mdx

@@ -41,6 +41,7 @@ The relevant required variables for each mode are noted in the descriptions belo
 |---|---|---|
 | `OSS_JWT_SECRET` | N/A | **Required for OSS deployments.** Secret used to sign JWT tokens. Must be set to a strong random value in production |
 | `OSS_JWT_EXPIRY_HOURS` | `720` | JWT token lifetime in hours (default: 30 days) |
+| `ENABLE_SIGNUP` | `true` | Set to `false` to disable public signup on invite-only installs — `POST /api/v1/auth/signup` returns 403 and the login page hides the Sign up link |
 
 <Warning>
 Never use the placeholder `OSS_JWT_SECRET` in a production deployment. Generate a strong random secret and store it securely.