Enable self-hosted telemetry by default

[suguanYang]

Summary

• enable anonymous self-hosted telemetry by default in shared runtime config
• hard-code Knowhere's PostHog US Cloud host and project key as built-in runtime defaults
• keep source quickstart env examples untouched: no telemetry host/key/id/enable configuration is shown to users
• add contract coverage for default-enabled telemetry and env override behavior

Verification

• uv run --python 3.11 --all-packages pytest apps/api/tests/contract/test_self_hosted_telemetry_contract.py -q
• make lint
• make typecheck
• uv run --all-packages --group lint ruff format --check apps/api/tests/contract/test_self_hosted_telemetry_contract.py packages/shared-python/shared/core/config/base.py
• git diff --check
• temporary local Gitleaks 8.30.1 arm64 binary: no leaks found