Add an admin-only server log viewer to the web UI

The root logger previously had no handler at all, so every module's
logger.info() call (user creation, claims, password resets, ...) was
silently dropped, not just unviewable. Adds a RotatingFileHandler
writing into the existing /data volume so log content also survives
container restarts/redeploys, plus /admin/logs (tail + line-count
picker + full-file download) alongside the existing Users & Frames
admin page.

server/tests/conftest.py

@@ -29,6 +29,10 @@ from pathlib import Path
 
 _tmp_dir = tempfile.mkdtemp(prefix="espresso_frame_tests_")
 os.environ["DATABASE_URL"] = f"sqlite:///{Path(_tmp_dir) / 'test.db'}"
+# Same reasoning as DATABASE_URL above: logging_setup.configure_logging()
+# also runs as an app.main import-time side effect and would otherwise
+# try to create the real /data directory.
+os.environ["LOG_PATH"] = str(Path(_tmp_dir) / "server.log")
 
 import pytest
 from fastapi.testclient import TestClient