fix: emit zero-value hits counter for active miss labels (OQP-2)

[TadMSTR]

Summary

• `oqp_embedding_cache_hits_total` emitted no samples when `cache_hits` was empty (e.g. after container restart), causing Prometheus to have no time series and the Grafana cache hit rate panel to show "No data" instead of 0%.
• Fix: after emitting real hit samples, iterate `cache_misses` and emit a `0` sample for any label not already in `cache_hits`.
• `cache_misses` always has entries after the first request, so the zero sentinel appears as soon as the cache is active.

Test plan

• Restart the proxy container (clears Valkey), send one embedding request, scrape `/metrics` — `oqp_embedding_cache_hits_total` should appear with value `0` for the active label.
• Send a second identical embedding request (cache hit), re-scrape — counter should increment to `1`.
• Grafana "Embedding Cache Hit Rate" panel should show `0%` immediately after restart rather than "No data".

Closes OQP-2

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