fix: set secure default for MODEL_CHANGE_PASSWORD

[AjayBandiwaddar]

Problem

MODEL_CHANGE_PASSWORD in app/config.py defaults to an empty
string when the environment variable is not set in .env.

The /change-model endpoint in app/routes/api.py checks:

if password != settings.MODEL_CHANGE_PASSWORD:
    raise HTTPException(status_code=403, detail="Invalid model change password")

This means on a fresh install without a .env file, anyone can
change the model by sending an empty password string — bypassing
the intended access control completely.

Fix

Changed the default value from "" to "changeme" so the
endpoint is protected even without explicit configuration.

Users should still set a strong MODEL_CHANGE_PASSWORD in
their .env file for production use.

Testing

Verified that the /change-model endpoint correctly rejects
requests with wrong passwords returning "Invalid model change password".

The fix addresses fresh installs where MODEL_CHANGE_PASSWORD
is not set in .env — previously defaulting to "" allowed
anyone to change the model by sending an empty password string.

[AjayBandiwaddar]

Problem

MODEL_CHANGE_PASSWORD in app/config.py defaults to an empty string when the environment variable is not set in .env.

The /change-model endpoint in app/routes/api.py checks:

if password != settings.MODEL_CHANGE_PASSWORD:
    raise HTTPException(status_code=403, detail="Invalid model change password")

This means on a fresh install without a .env file, anyone can change the model by sending an empty password string — bypassing the intended access control completely.

Fix

Changed the default value from "" to "changeme" so the endpoint is protected even without explicit configuration.

Users should still set a strong MODEL_CHANGE_PASSWORD in their .env file for production use.

Testing

Verified that the /change-model endpoint correctly rejects requests with wrong passwords returning "Invalid model change password".

The fix addresses fresh installs where MODEL_CHANGE_PASSWORD is not set in .env — previously defaulting to "" allowed anyone to change the model by sending an empty password string.

Problem

MODEL_CHANGE_PASSWORD in app/config.py defaults to an empty string when the environment variable is not set in .env.

The /change-model endpoint in app/routes/api.py checks:

if password != settings.MODEL_CHANGE_PASSWORD:
    raise HTTPException(status_code=403, detail="Invalid model change password")

This means on a fresh install without a .env file, anyone can change the model by sending an empty password string — bypassing the intended access control completely.

Fix

Changed the default value from "" to "changeme" so the endpoint is protected even without explicit configuration.

Users should still set a strong MODEL_CHANGE_PASSWORD in their .env file for production use.

Testing

Verified that the /change-model endpoint correctly rejects requests with wrong passwords returning "Invalid model change password".

The fix addresses fresh installs where MODEL_CHANGE_PASSWORD is not set in .env — previously defaulting to "" allowed anyone to change the model by sending an empty password string.

Note: This PR was developed with AI assistance (Claude). As per Sugar Labs contributing guidelines, I'm disclosing this. The AI helped me structure the fix, but I verified, and tested each change myself.