Remove or implement langfuse backend mentioned in config

[Fuabioo]

Problem

The config documentation mentions a "langfuse" backend option, but no implementation exists.

Evidence

config.go:29

// Backend specifies the sync backend ("console", "langfuse").
Backend string `yaml:"backend"`

However, examining the internal/backend/ directory shows only:

• backend.go
• console.go
• console_test.go

Using backend: langfuse in the config file produces an "unknown backend: langfuse" error at runtime.

Impact

Users may try to configure langfuse based on the comment and encounter a confusing error.

Suggested Fix

Either:

1. Remove the langfuse mention from the comment if there are no plans to implement it
2. Implement the langfuse backend if this is a planned feature (and document it properly)

Files: internal/config/config.go:29