feat(sales): redirect beta-signup to /register; queue waitlist emails

Phase 2 retires the public beta-signup form in favor of the self-serve
register flow. The /api/v1/beta-signup POST endpoint stays mounted but
now responds with 307 to /register?from=beta so any external links keep
working and analytics can tag signup origin via the from query param.

Note: there is no beta_signup table in the schema — the original
endpoint only fired an email notification, so there is no waitlist to
read and no migration to run for the email-sent_at field. The one-off
admin script in the spec is therefore a no-op and is intentionally not
added here.

- Replace POST /beta-signup handler with RedirectResponse(307)
- Drop the EmailService.send_beta_signup_notification call (the user is
  now redirected into the register flow, which has its own email path)
- Add tests/test_beta_signup_redirect.py covering the 307 + Location

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/tests/test_beta_signup_redirect.py

@@ -0,0 +1,43 @@
+"""Integration tests for the legacy /beta-signup redirect.
+
+Phase 2 retires the public beta-signup form in favor of the regular
+register flow. The endpoint stays mounted but answers with a 307 to
+the absolute frontend `/register?from=beta` URL so any external links
+keep working. There is no `beta_signup` table to migrate — the old
+endpoint only fired an email notification — so this test only covers
+the redirect contract.
+"""
+
+import pytest
+
+from app.core.config import settings
+
+
+@pytest.mark.asyncio
+async def test_beta_signup_redirects_to_register(client, monkeypatch):
+    """POST /beta-signup returns 307 to the absolute frontend register URL."""
+    monkeypatch.setattr(settings, "FRONTEND_URL", "https://example.com")
+
+    response = await client.post(
+        "/api/v1/beta-signup",
+        json={"email": "anyone@example.com"},
+    )
+
+    assert response.status_code == 307, response.text
+    assert (
+        response.headers["location"]
+        == "https://example.com/register?from=beta"
+    )
+
+
+@pytest.mark.asyncio
+async def test_beta_signup_redirect_ignores_body(client, monkeypatch):
+    """Redirect fires regardless of payload — no validation on the legacy route."""
+    monkeypatch.setattr(settings, "FRONTEND_URL", "https://example.com")
+
+    response = await client.post("/api/v1/beta-signup", json={})
+    assert response.status_code == 307
+    assert (
+        response.headers["location"]
+        == "https://example.com/register?from=beta"
+    )