Extract postcode districts & fetch districts instead when bbox is too big

[rm-hull]

No description provided.

[gemini-code-assist]

Summary of Changes

Hello @rm-hull, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the data extraction utility by introducing support for processing and storing GeoJSON data related to postcode districts, alongside the existing postcode units. This change is foundational for enabling broader search capabilities, particularly when dealing with large geographical areas, as suggested by the PR title's mention of 'searching when bbox is too big'. The modifications generalize the data handling pipeline to accommodate different types of geographical data.

Highlights

• Support for Postcode Districts: The data extraction utility (cmd/extract_data.go) has been updated to explicitly handle and store GeoJSON data for postcode districts. This includes creating dedicated directories (./data/postcodes/districts) for these new data types.
• Generalized Data Processing Logic: The core logic for identifying and reprocessing GeoJSON features from the input tarball has been refactored. A new extractFileType function now dynamically determines if a file represents a 'unit' or 'district' and identifies the relevant property name (e.g., 'postcodes' or 'district') within the GeoJSON feature. The reprocessFeatureCollection function was updated to leverage this, making it more generic for handling both data types.
• Dynamic Output File Structure: Processed and compressed GeoJSON files are now saved into type-specific subdirectories. For example, postcode units go into ./data/postcodes/units/ and postcode districts go into ./data/postcodes/districts/, ensuring better organization of the extracted data.

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[github-actions]

	Tests	Passed ❌️	Skipped	Failed
JUnit Test Report	0 ran	0 passed	0 skipped	0 failed

Test	Result
No test annotations available

• View JUnit Test Report

[gemini-code-assist]

Code Review

The code changes introduce the ability to extract postcode districts in addition to postcode units, and refactors the code to handle both types of data. The changes also allow the program to continue processing even if some files fail to extract or compress. The review focuses on error handling and improving the robustness of the extraction process.