Enhance: fix Show BTS within distance query, parameter validation, refactor code, update README

[hch155]

Fix Show BTS within distance query in app.py

• When a valid max_distance is provided, limit is automatically set to None. This prioritizes distance-based queries, facilitating a broader overview of base stations within a specified range

Parameter validation in app.py

Added validation checks to ensure:

• max_distance falls within 0.1 to 10 km,
• limit does not exceed 10,

preventing unrealistic range queries that could affect performance / exploit app and the responses are within expected data thresholds.

Refactor code in queries.py

• remove redundant duplicate line that sorted final_stations_list

Update README

• add application URL