refactor(backend): clean up models setup, improve ip utils, add adr docs

- Extract ADR documents for architectural decisions (SQLite, FastAPI, React, APScheduler, Scheduler)
- Refactor setup.py: improve code structure and readability
- Add IP validation utilities with test coverage
- Update frontend components (BanTable, HistoryPage)
- Add pre-commit hooks and CONTRIBUTING.md
- Add .editorconfig for consistent coding standards

Docs/adr/ADR-002-FastAPI-Backend-Framework.md

@@ -0,0 +1,45 @@
+# ADR-002: FastAPI over Django
+
+## Status
+Accepted
+
+## Context
+The backend requires a Python async web framework with strong typing, validation,
+and OpenAPI support.
+
+## Decision
+Use **FastAPI** as the backend framework.
+
+## Rationale
+
+### Why FastAPI over Django?
+- **Async-first:** FastAPI is built on Starlette with native `async def` route
+  handlers. Django's ORM and request handling are synchronous, requiring thread
+  pools for I/O-bound work.
+- **Modern Python 3.12+:** FastAPI embraces modern Python idioms — type annotations,
+  structural pattern matching, dataclasses. Django maintains broad Python 3.8+
+  compatibility and shows its age.
+- **Pydantic v2 integration:** FastAPI natively uses Pydantic for request/response
+  validation. Automatic OpenAPI schema generation from Pydantic models is seamless.
+- **Dependency injection:** FastAPI's `Depends()` system provides a lightweight,
+  explicit DI pattern without a separate container library.
+- **Performance:** FastAPI + Uvicorn consistently benchmarks as one of the fastest
+  Python web frameworks, comparable to Node.js and Go for JSON APIs.
+
+### Why not Django?
+- Django's synchronous ORM creates thread-pool bottlenecks with SQLite.
+- Django's "batteries-included" philosophy is overkill for BanGUI's scope.
+  We need REST endpoints and background tasks, not a full CMS.
+- Less flexible dependency injection — Django's middleware and view system is
+  less composable than FastAPI's routing layers.
+
+### Trade-offs
+- **Smaller ecosystem:** Django has decades of third-party packages. FastAPI's
+  ecosystem is younger but covers BanGUI's needs (structlog, aiosqlite, APScheduler,
+  aiohttp) completely.
+- **No built-in admin UI:** BanGUI is its own admin UI; Django's admin is irrelevant.
+
+## Consequences
+- FastAPI routes are defined in `app/routers/`.
+- Request/response models live in `app/models/`.
+- Dependency injection via `app/dependencies.py`.