feat: add Langfuse tracing integration

[Suman-20]

Summary

• Added Langfuse tracing integration to the LiveKit AI voice agent.
• Added a dedicated telemetry setup for Langfuse.
• Added session-level tracing using the LiveKit room name.
• Added trace flushing during agent shutdown.
• Verified that agent and LLM traces are visible in Langfuse.

Issue Or Context

• Related issue: N/A
• First-time contributor?: Yes
• Area changed: AI worker / telemetry

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:

python main.py console --input-device 1