Feature/langfuse integration

[harsh-jain-19]

Summary

Issue Or Context

• Related issue:
• First-time contributor?:
• Area changed: README, docs, web app, console, API, AI worker, telephony, billing, or local tooling

Verification

• I ran the narrowest check that proves this change.
• I ran task doctor when local tooling, env files, Docker services, or setup docs changed.
• I ran pnpm ci:local for shared code, CI, dependency, build, or release workflow changes, or documented the narrower check below.
• Dependency changes are intentional, reflected in pnpm-lock.yaml, and any audit suppression changes include an expiry date.
• UI screenshots or recordings are attached for product UI changes, or this PR has no product UI surface.
• Environment changes are documented in the relevant .env.*.example, workflow variable notes, or README section.
• I added broader checks if this touches shared code, auth, billing, database models, runtime agent behavior, or production UI.
• For docs-only changes, I reviewed links, commands, and provider-credential boundaries.

Command or review evidence:

Launch And Positioning Guardrails

• Public copy keeps the open-source Retell alternative story focused on control, self-hosting, privacy review, cost visibility, and extensibility.
• Competitive comparisons explain tradeoffs without attacking competitors.
• This PR does not invent screenshots, metrics, benchmarks, customers, compliance claims, provider partnerships, or production readiness claims.
• Provider requirements are explicit when real calls, billing, OAuth, email, storage, or deployment need live credentials or product decisions.

Screenshots Or Recordings

Add only real screenshots or recordings captured from the current product or a documented local/demo environment.

Maintainer Review Notes

• Is this safe for a first-time contributor path, or does it need owner review because it touches credentials, auth, billing, migrations, runtime workers, or provider architecture?
• If this is a launch-day PR, provide a first response within 24 launch hours even when full review needs more time.