service configure upgrade & API simplification

[ronibhakta1]

This pull request updates the /service API and related documentation to surface provider-level default quality and controls fields, making it easier for clients to determine what features each provider supports (including unsupported features, which are now explicitly marked as false). The code, schema, tests, and docs have all been updated to reflect this new shape.

API and Schema Changes:

• The /service endpoint now includes quality and a full controls map (with both enabled and disabled features) for each provider, allowing clients to see what each provider is capable of, not just what's enabled for individual voices. [1] [2] [3] [4]

Implementation Changes:

• Added an as_dict method to the Controls class in app/schemas/voice.py to serialize all control features (enabled and disabled) for use in the /service response.
• Updated provider capabilities construction to use provider defaults for quality and controls.

Documentation Updates:

• Updated docs/API.md to document the new quality and controls fields on /service, with an explanation of the difference between provider-level and voice-level controls, and an updated example response. [1] [2]
• Clarified error code documentation for 503 service_not_ready with more detailed causes.
• Improved configuration and provider documentation for clarity and accuracy. [1] [2]

Testing:

• Updated tests to assert the new shape of provider capabilities, including the presence and type of the controls and quality fields.

Other:

• Added a License section to the README.md.