fix: standardize Langfuse endpoint on LANGFUSE_BASE_URL

[norberia]

Use the official LANGFUSE_BASE_URL environment variable for Chat Assistant tracing and forwarding into newly created GitHub Deploy Devboxes. Update Helm values, environment examples, Turbo configuration, tests, and migration notes together.

This is a breaking configuration rename: LANGFUSE_HOST is no longer read or forwarded. Deployments must rename the variable and recreate existing Devboxes to apply the new environment. Historical changelog entries retain their original names.

Validation:

• 47 focused tests passed, including rejection of the old variable and Devbox forwarding coverage.
• bun typecheck passed.
• bun lint passed. The removed-variable regression test has a scoped Turbo lint exemption because the old variable intentionally is not declared in turbo.json.
• bun check passed (1,660 files).
• git diff --check passed.

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-10T09:53:20.527888Z	efbec24	PR opened

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