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-001-SQLite-Application-Database.md

@@ -0,0 +1,45 @@
+# ADR-001: SQLite as the Application Database
+
+## Status
+Accepted
+
+## Context
+BanGUI needs a database to store application state: configuration, session records,
+blocklist sources, import logs, and ban history archives.
+
+## Decision
+Use **SQLite** (via `aiosqlite`) as BanGUI's application database, persisted to a
+volume mounted from the host or a named Docker volume.
+
+## Rationale
+
+### Why SQLite over PostgreSQL?
+- **Zero-infrastructure:** No separate DB server process, no connection pooling,
+  no credentials to manage. Ships in the Docker container with no additional
+  configuration.
+- **Fail2ban-compatible:** The fail2ban database itself is SQLite. BanGUI already
+  depends on SQLite; adding a second database engine would increase operational
+  complexity for no benefit.
+- **Single-instance deployment:** BanGUI runs as a single service with one
+  background scheduler (enforced by `BANGUI_WORKERS=1`). Horizontal scaling is not
+  a design goal.
+- **Async I/O:** `aiosqlite` provides full async support, avoiding blocking I/O in
+  the FastAPI async request handlers.
+
+### Why not PostgreSQL?
+- Requires a separate service or sidecar container.
+- Adds connection overhead (TCP, connection pools, auth).
+- Over-engineered for a single-instance web app.
+
+### Trade-offs
+- **Not suitable for multi-worker deployments.** SQLite's file-level locking means
+  only one process can write at a time. This is explicitly enforced:
+  `BANGUI_WORKERS=1` is validated at startup, and `scheduler_lock` prevents
+  duplicate schedulers in restarts.
+- For future multi-instance deployments, BanGUI would need to migrate to
+  PostgreSQL or add a distributed lock layer (Redis + job store).
+
+## Consequences
+- Application database is a single file (`bangui.db`) in the container's data volume.
+- Backup is a file copy. No `pg_dump` equivalent needed.
+- Schema migrations managed via `app/startup.py` startup DAG.