fix: change appname to neoirc and default DB to /var/lib/neoirc/state.db

- Change Appname from "chat" to "neoirc" in cmd/chatd/main.go
- Change default DBURL from file:./data.db to file:///var/lib/neoirc/state.db
  in both internal/config/config.go and internal/db/db.go fallback
- Create /var/lib/neoirc/ directory in Dockerfile with proper ownership
- Update README.md to reflect new defaults (DBURL, docker run example,
  SQLite backup/location docs)
- Remove stale data.db reference from Makefile clean target

The DB path remains configurable via DBURL environment variable.

Closes #44

internal/config/config.go

@@ -56,7 +56,7 @@ func New(
 	viper.SetDefault("DEBUG", "false")
 	viper.SetDefault("MAINTENANCE_MODE", "false")
 	viper.SetDefault("PORT", "8080")
-	viper.SetDefault("DBURL", "file:./data.db?_journal_mode=WAL")
+	viper.SetDefault("DBURL", "file:///var/lib/neoirc/state.db?_journal_mode=WAL")
 	viper.SetDefault("SENTRY_DSN", "")
 	viper.SetDefault("METRICS_USERNAME", "")
 	viper.SetDefault("METRICS_PASSWORD", "")