Memory coverage: users-with-memory / active-end-users (clamped 100%)

Single-object response for /v1/analytics/memory/coverage.

Numerator (users_with_memory and the episode counts) comes from Tinybird; total_end_users comes from Neo4j. The service merges both, computing coverage_rate_pct and clamping at 100 for the edge case where a user was deleted after their memory was synced.