fix: change appname to neoirc, default DB to /var/lib/neoirc/state.db (#45)

## Changes

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

The DB path remains configurable via the `DBURL` environment variable. No Go packages were renamed.

Closes #44

Co-authored-by: clawbot <clawbot@noreply.eeqj.de>
Reviewed-on: #45
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

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(