Add navigation & usage metrics for end-user experience visibility
Instrument DB query timing (11 operations across 3 repositories),
streaming lifecycle (TTFB, duration, feature count), cache operation
latency, listing detail step breakdown, and frontend page load /
time-to-first-listing / stream download / detail load metrics.
Adds 16 new OTel instruments, extends the perf ingestion endpoint
with 4 new frontend metrics, and adds ~20 Grafana dashboard panels
across 4 new rows (DB Query Performance, Streaming Performance,
Listing Detail Breakdown, Cache Performance, Frontend Navigation).
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