Make the per-owner project cap configurable via env

Self-hosters shouldn't be stuck with a hardcoded 100-project limit;
MAX_PROJECTS_PER_OWNER now controls it, defaulting to 0 (unlimited).

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

docs/setup.md

@@ -56,6 +56,7 @@ All settings are optional environment variables (read from `.env` or the real en
 | `JWT_TTL` | `86400` | Token lifetime in seconds |
 | `APP_ALLOW_REGISTRATION` | `true` | When `false`, a magic link is only ever sent to an existing address — an unknown one is silently ignored, so no new accounts get created |
 | `MAGIC_LINK_RESEND_SECONDS` | `60` | Minimum gap before a magic link can be resent to the same address (sign-in or email-change). Docker Compose overrides this to `0`, so links resend immediately in development |
+| `MAX_PROJECTS_PER_OWNER` | `0` | Maximum number of projects a single user may create. `0` means unlimited |
 | `APP_URL` | `http://localhost:8080` | Base URL used to build magic links (`http://localhost:5173` for a host `npm run dev`) |
 | `WEBAUTHN_RP_ID` | `APP_URL`'s host | Passkey relying party ID (domain). Must be `localhost` or a real domain over HTTPS — a LAN IP won't work |
 | `WEBAUTHN_RP_NAME` | `Projects` | Passkey relying party display name, shown in the browser/OS prompt |