Issue 136 - Add "optimap" and timestamp to the download files names

[BharatVe]

Closes #136 #145

1. Helpers

• Added generate_data_dump_filename(ext) → optimap_data_dump_.
• Added cleanup_old_data_dumps(dir, keep) to prune old files beyond the retention count

Tasks (publications/tasks.py)

• Swapped out hard-coded names in regenerate_geojson_cache/regenerate_geopackage_cache for the new helpers
• Call cleanup after each dump
• Fixed cache_dir resolution in geopackage task

2. Config

• .env.example: OPTIMAP_DATA_DUMP_RETENTION=3
• settings.py: loads into DATA_DUMP_RETENTION (default 3)

Views (publications/views.py)

• data() now globs for the latest optimap_data_dump_*.{geojson,gpkg}, humanizes size, and computes “Last updated”
• download_geojson()/download_geopackage() regenerate and serve the actual timestamped filenames

3. Tests (tests/test_geo_data.py)

• Import fixes (tempfile, gzip)
• Tests glob for optimap_data_dump_* instead of static names
• New tests for the filename generator and cleanup logic

4. Add log statement

• Gather the modification times of the GeoJSON, GZIP and GPKG dumps and emit a logger.warning if they differ by more than one hour.

[nuest]

Well done.