Refactor error handling to use Problem Details format across API responses

[ronibhakta1]

Closes #5
This pull request refactors the API error handling to fully adopt the RFC 9457 "problem+json" standard for error responses, improving consistency and machine-readability. It introduces a new ProblemDetail schema, updates all error handlers and OpenAPI docs to use this format, and adjusts tests and endpoints accordingly. Additionally, it adds a Makefile help target and improves documentation for developer workflows.

API error handling improvements:

• Introduced a new ProblemDetail model and problem_response helper in app/api/errors.py to standardize all error responses as application/problem+json per RFC 9457.
• Updated all custom exceptions to include a title field and mapped them to appropriate HTTP status codes and error URIs. [1] [2]
• Refactored all error handlers and endpoints (including /readyz, /synthesize, /voices) to return problem+json responses and updated OpenAPI documentation to reference the new schema. [1] [2] [3] [4] [5] [6] [7]
• Enhanced validation error responses to include detailed field errors in the errors field.

Test updates:

• Updated and expanded tests to validate the new error response format, including type, title, status, instance, and errors fields. [1] [2] [3]

Developer workflow improvements:

• Added a help target to the Makefile as the default goal, and added/updated descriptions for all targets to improve developer experience.