Add heatmap push command for city tracks

[maltehuebner]

Summary

• Adds new Symfony command criticalmass:heatmap:push-city-tracks that takes a city slug as argument
• Collects all high-resolution polylines from a city's tracks and pushes them to the maps.jetzt heatmap API
• Creates a heatmap with identifier criticalmass-{slug} (e.g. criticalmass-hamburg)
• Sends polylines in batches of 50 with progress bar output
• Handles existing heatmaps gracefully (HTTP 409) by continuing to add polylines

Test plan

• Run bin/console criticalmass:heatmap:push-city-tracks hamburg with a valid city slug
• Verify heatmap is created on maps.jetzt with correct identifier
• Verify polylines are sent as high-resolution (not reduced) encoded polylines
• Test with invalid slug — should show error message
• Test with city that has no tracks — should show warning
• Run command twice — second run should handle 409 and still push polylines

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