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/app/db.py

@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+from pathlib import Path
 from typing import TYPE_CHECKING
 
 from langgraph.checkpoint.postgres.aio import AsyncPostgresSaver
@@ -88,6 +89,17 @@ async def create_checkpointer(pool: AsyncConnectionPool) -> AsyncPostgresSaver:
     return checkpointer
 
 
+def run_alembic_migrations(database_url: str) -> None:
+    """Run Alembic migrations to head."""
+    from alembic.config import Config
+
+    from alembic import command
+
+    alembic_cfg = Config(str(Path(__file__).parent.parent / "alembic.ini"))
+    alembic_cfg.set_main_option("sqlalchemy.url", database_url)
+    command.upgrade(alembic_cfg, "head")
+
+
 async def setup_app_tables(pool: AsyncConnectionPool) -> None:
     """Create application-specific tables and apply migrations."""
     async with pool.connection() as conn: