fix: user-defined providers take precedence over built-in providers

[octo-patch]

Fixes #220

Problem

GetProvider() in backend/pkg/providers/providers.go checked built-in providers first via a switch statement on the provider name. If the user-defined provider shared a name with a built-in one (e.g. "qwen"), the switch matched and returned the built-in provider, ignoring the user's custom configuration in the database.

Solution

Reverse the lookup order in GetProvider():

1. Query the database for a user-defined provider with the given name first.
2. If found (sql.ErrNoRows is not returned), build and return that provider — it takes precedence.
3. Only if no user-defined provider exists (sql.ErrNoRows), fall back to the built-in provider switch.

This ensures that user-configured providers always override the defaults, regardless of whether the name matches a built-in provider.

Testing

Manually verified the logic flow:

• When a user-defined provider named "qwen" exists in the DB, GetUserProviderByName returns it and the function returns the user-built provider without entering the switch.
• When no user-defined provider exists for a name, sql.ErrNoRows causes the function to fall through to the built-in switch, preserving existing behavior for all other provider names.

[salecharohit]

#233 related ?

[asdek]

@octo-patch Thank you for the PR!