refactor: engineering improvements -- API versioning, structured logging, Alembic, error standardization, test coverage

- API versioning: all REST endpoints prefixed with /api/v1/
- Structured logging: replaced stdlib logging with structlog (console/JSON modes)
- Alembic migrations: versioned DB schema with initial migration
- Error standardization: global exception handlers for consistent envelope format
- Interrupt cleanup: asyncio background task for expired interrupt removal
- Integration tests: +30 tests (analytics, replay, openapi, error, session APIs)
- Frontend tests: +57 tests (all components, pages, useWebSocket hook)
- Backend: 557 tests, 89.75% coverage | Frontend: 80 tests, 16 test files

backend/tests/unit/test_logging_config.py

@@ -0,0 +1,20 @@
+"""Tests for structured logging configuration."""
+
+from __future__ import annotations
+
+import pytest
+
+from app.logging_config import configure_logging
+
+
+pytestmark = pytest.mark.unit
+
+
+def test_configure_logging_console_mode() -> None:
+    """Console mode configures without error."""
+    configure_logging("console")
+
+
+def test_configure_logging_json_mode() -> None:
+    """JSON mode configures without error."""
+    configure_logging("json")