Add design spec for env-var-controlled sign-up

docs/specs/2026-08-19-allow-signup-env-design.md

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+# Allow Sign-Up Via Environment Variable
+
+Date: 2026-08-19
+Status: Draft
+
+## Summary
+
+Replace the current "count of users" check that gates sign-up with a
+single environment variable. This lets an operator explicitly control
+whether public sign-up is available, instead of Podstalk inferring it
+from how many users already exist.
+
+## Motivation
+
+Today `ServeSignup` disables sign-up once a user already exists
+(`store.UserCount() > 0`). That is implicit and easy to get wrong: the
+first user must register before anyone can log in, and once that happens
+no one can ever register again. Controlling this with an explicit
+environment variable is predictable and self-documenting.
+
+## Behavior
+
+New environment variable: `PODSTALK_ALLOW_SIGNUP`.
+
+- Unset or any falsy value (`false`, `0`, `no`, `off`): sign-up **disabled**.
+- Truthy value (`true`, `1`, `yes`, `on`): sign-up **enabled**.
+
+Default (unset) is **disabled**.
+
+## Changes
+
+### `main.go`
+
+Read `PODSTALK_ALLOW_SIGNUP` from the environment and resolve it with a
+truthy-value parser. Set the resulting boolean on the `AuthHandler`.
+
+### `handler/auth.go`
+
+- Add an `AllowSignup bool` field to `AuthHandler`.
+- `ServeSignup`: redirect to `/signin` when `!AllowSignup` (replaces the
+  `UserCount() > 0` check).
+- Replace the `hasUsers()` helper (driven by user count) with a check on
+  `AllowSignup`, so the Sign In page shows/ hides the "Sign Up" nav link
+  based on whether sign-up is enabled.
+
+### Unchanged
+
+`store.UserCount()` remains in the store package. It is no longer used by
+authentication but is left in place as a general utility.
+
+## Out of Scope
+
+- No admin/user-management UI to invite specific users.
+- No change to the existing `adduser` CLI command (operator provisioning
+  still works regardless of the sign-up toggle).
+- No change to sign-in/sign-out behavior.
+
+## Testing
+
+- `scripts/build` passes (gofmt, go vet, go build, docker build).
+- Manual verification is limited by the sandbox; behavior relies on the
+  straightforward boolean branch and the truthy parser.