Add anonymous self-hosted telemetry client

[suguanYang]

Summary

• add self-hosted telemetry config fields to shared API settings
• add a small anonymous PostHog batch client using existing httpx
• generate a stable anonymous installation id from env or a persistent file
• emit API startup, heartbeat, and shutdown events without user/person profiles
• enforce event/property allowlists so private request/document/query fields are stripped

Privacy boundary

• PostHog events use the anonymous installation id as distinct_id
• events include $process_person_profile=false
• no user id, email, org id, IP address, document id, chunk id, filename, prompt, response, retrieval query text, request body, API key, webhook URL, or raw stack trace is emitted

Tests

• uv run pytest apps/api/tests/contract/test_self_hosted_telemetry_contract.py -q
• make lint
• make typecheck
• git diff --check

[EricNGOntos]

LGTM - all CI checks passed, no conflicts with main.