Support downloading premium exercises with API key (#2)

* feat: add API key support to exercise downloader

Read PYBITES_API_KEY env var and send X-API-Key header when set.
Without it, only free exercises are downloaded.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* docs: add API key instructions to README

Explain how to set PYBITES_API_KEY for downloading all premium exercises.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* refactor: extract write_all_exercises and auth_status_message for testability

Pull the exercise-writing loop and auth status message out of main()
into standalone functions that can be unit tested.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* test: add tempfile and serde_json dev dependencies for unit tests

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* update gitignore with default exercises dir created running it locally

* refactor: restore essential comments and move println into write_all_exercises

Restore two comments removed during refactoring that explain *why*
(--test flag purpose, backup rationale for solved exercises). Move
per-exercise println into write_all_exercises for consistency.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: bbelderbos

.gitignore

@@ -23,3 +23,4 @@ target
 ### custom section ###
 
 notes/
+exercises

README.md

@@ -10,10 +10,19 @@ Exercise downloader for https://rustplatform.com/
     cargo install --git https://github.com/markgreene74/pybites_rust.git
     ```
 - `cd` to the directory where you want to save the exercises
-- run the downloader
+- run the downloader (free exercises only):
     ```shell
     pybites-rust-download
     ```
+- to download **all** exercises (requires premium), set your API key:
+    ```shell
+    PYBITES_API_KEY=your-api-key-here pybites-rust-download
+    ```
+    Or export it in your shell profile so you don't have to pass it every time:
+    ```shell
+    export PYBITES_API_KEY=your-api-key-here
+    ```
+    You can find your API key on your [profile page](https://rustplatform.com/profile/).
 
 ### Compile it manually
 

exercise_downloader/Cargo.lock

@@ -10,3 +10,7 @@ serde = { version = "1.0.227", features = ["derive"] }
 [[bin]]
 name = "pybites-rust-download"
 path = "src/main.rs"
+
+[dev-dependencies]
+serde_json = "1.0.149"
+tempfile = "3.25.0"