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>

.env.example

@@ -29,6 +29,9 @@ APP_ALLOW_REGISTRATION=true
 # to 0 for local development, so links can be resent immediately.
 MAGIC_LINK_RESEND_SECONDS=60
 
+# Maximum number of projects a single user may create. 0 means unlimited.
+MAX_PROJECTS_PER_OWNER=0
+
 # Base URL the app is reached at. Verification magic links point here, e.g.
 # <APP_URL>/verify-email?token=... The Docker image serves the SPA and the API
 # together on http://localhost:8080; a host `npm run dev` serves it on :5173.