fix: added missing historical datasets + external ids

[cka-y]

Summary:

This pull request adds a helper function to ensure that every feed has a properly derived TransitFeeds external ID, and integrates this check into the historical dataset import process. The main goal is to improve data consistency by guaranteeing that each feed is associated with a unique external identifier from TransitFeeds.

TransitFeeds external ID management:

• Added the _ensure_transitfeeds_externalid function to derive and attach an Externalid with source='transitfeeds' to a feed, based on the TransitFeeds dataset ID. This function checks for existing IDs to avoid duplicates and logs actions for traceability.
• Integrated the new external ID check into the _add_historical_datasets function, ensuring that the external ID is set for each feed before processing historical datasets.

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[davidgamez]

LGTM!