Invite passphrase for account creation

Date: 2026-04-04

Added a lightweight gate to account creation while the app is in early access, before email verification exists.

Set INVITE_PASSPHRASE=<phrase> in the backend environment. When set:

• POST /auth/signup checks that invite_code in the request body matches the passphrase, returning 403 if it doesn't
• GET /v1/config returns {"invite_required": true} so the client knows to show the field
• The Flutter signup screen fetches /v1/config on mount and conditionally renders an "Invite Code" field

When the env var is absent, no check is performed and the field is hidden — zero friction for local development.

A few implementation details worth noting:

Constant-time comparison — the passphrase check uses crypto/subtle.ConstantTimeCompare instead of !=. A regular string comparison short-circuits on the first differing byte, which in theory leaks timing information about how many leading characters the attacker got right. Constant-time comparison always scans the full string. No added latency; it's just the correct primitive for secret comparisons.

Config endpoint — rather than embedding the invite flag in some other response, a dedicated GET /v1/config endpoint was added. It's public, unauthenticated, and currently returns only invite_required. It's a natural place to add other client-facing feature flags later without breaking anything.