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/db/db.go

@@ -87,7 +87,7 @@ func (database *Database) GetDB() *sql.DB {
 func (database *Database) connect(ctx context.Context) error {
 	dbURL := database.params.Config.DBURL
 	if dbURL == "" {
-		dbURL = "file:./data.db?_journal_mode=WAL&_busy_timeout=5000"
+		dbURL = "file:///var/lib/neoirc/state.db?_journal_mode=WAL&_busy_timeout=5000"
 	}
 
 	database.log.Info(