refactor(tests): compress admin_client + mock_send_email into conftest

Lift admin_client fixture from 7 duplicate definitions into conftest.py.
Add mock_send_email fixture, replacing 60 inline patch() blocks across
test_emails.py, test_waitlist.py, and test_businessplan.py. Net -197 lines.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

web/tests/test_seo.py

@@ -89,26 +89,6 @@ async def articles_data(db, seo_data):
     await db.commit()
 
 
-@pytest.fixture
-async def admin_client(app, db):
-    """Authenticated admin client."""
-    now = utcnow_iso()
-    async with db.execute(
-        "INSERT INTO users (email, name, created_at) VALUES (?, ?, ?)",
-        ("admin@test.com", "Admin", now),
-    ) as cursor:
-        admin_id = cursor.lastrowid
-    await db.execute(
-        "INSERT INTO user_roles (user_id, role) VALUES (?, 'admin')", (admin_id,)
-    )
-    await db.commit()
-
-    async with app.test_client() as c:
-        async with c.session_transaction() as sess:
-            sess["user_id"] = admin_id
-        yield c
-
-
 # ── Query function tests ─────────────────────────────────────
 
 class TestSearchPerformance: