Switch web framework from Echo to Chi, removing Echo-related
dependencies
and adding chi and cors. Update Stripe to v83.1.0 and Sentry to v0.36.2.
Remove unused and indirect dependencies for a cleaner go.mod/go.sum.
This commit standardizes the naming of identifier and API key fields
across the codebase to use consistent camel case (e.g., `ID`, `APIKey`,
`DatabaseURL`). This includes updates to struct fields, method names,
function parameters, and environment variable references. The changes
improve code clarity and maintainability by reducing ambiguity and
aligning with Go naming conventions. No functional behavior is changed.
