Read and write street-tree plot and tree records (species, diameter, location, stewardship) in an OpenTreeMap instance via the OTM v4 API with HMAC-signed requests

Steps

1. Obtain an ACCESS_KEY/SECRET_KEY pair — either create one via APIAccessCredential in a self-hosted OTM Django shell, or request one from the hosting provider for a hosted opentreemap.org instance
2. Sign every request per the documented HMAC scheme: build a sign string from HTTP verb, host header, request path, sorted query params, and body, then HMAC-SHA256 it with the secret key and attach access_key/timestamp/signature as query params
3. Call GET /instance/{instance-url-name}/locations/{lat},{lon}/plots to find planting sites near a point (tune max_plots and distance)
4. POST a JSON body with plot.geom (and optional tree.diameter, etc.) to /instance/{instance-url-name}/plots to create a new planting site and tree
5. PUT to /instance/{instance-url-name}/plots/{plot-id} with a partial JSON body to update an existing plot or tree's fields
6. Call GET /instance/{instance-url-name}/species to resolve valid species IDs before writing tree.species