Add a "Dataset Importer" Windmill app

[rudokemper]

Goal

Closes #117.

Screenshots

What I changed

• Added f/apps/gc_dataset_importer.app, which consists of:
  • A Windmill UI (defined in app.yaml) built around the Stepper component to guide the user through uploading a dataset.
  • A set of Python scripts to handle database interactions, file uploads, conversions, transformations, and writing to the database.
  • A set of frontend scripts to manage reactive state across the app.
• Updated code previously merged under the GC Dataset Importer epic -- specifically in data_conversion.py and file_operations.py. I’ve added comments and additions to the docstrings to clarify why these changes were necessary.
• Deleted the prototype f/apps/gc_uploader.app, which only supported Locus Map data.
• I tried to make the app README as helpful as possible, usage examples, a diagram, and more. Please let me know if you think any of it is superfluous.

Notes to the reviewer

• The easiest way to review this app is to explore it in a Windmill instance in the edit view (e.g. /apps/edit/f/apps/gc_dataset_importer) and walk through the steps to see how scripts are called, how state is managed, etc.
• In terms of the code review, I'd recommend looking at the Python scripts in f/apps/gc_dataset_importer.app, and any changes to f/common_logic.
• On that note: in building this, I tried to be mindful of where the code should live. The three Python scripts in f/apps/gc_dataset_importer.app mainly call functions from imported modules, handle errors, and log outcomes. Any broader business logic that I felt could be useful to other code was added to the appropriate file in f/common_logic/. There are nevertheless a few helper functions in the Python scripts in the app directory since they are highly specific to the app, so I did not think it was worth moving them into f/common_logic/. Opinions on my choices are welcome.
• As it stands, there are some changes I made to f/connectors/ code in this branch. I already filed those as separate PRs -- Add CSV to Postgres script #135 and Generate unique ID if not present in GeoJSON feature #136. After these are merged, I will rebase on main so that overlapping changes here are dropped.
• There was a ton of iteration involved in building this, so I don't think the commit history is worth looking at. (As is customary in this repo, will squash and merge).

What I'm not doing here

• A bunch of stuff that came up during review that I felt was either out of scope, or better off as a separate improvement due to the complexity involved. These are listed in a TODO section in the README.