apollo_dashboard: alert on frozen exchange-rate oracle (eth_to_strk, strk_to_usd) (#14706)

[matanl-starkware]

Backport of #14706 to main-v0.14.3.

Clean git cherry-pick -x of a620b9af39. No starknet_version change (V0_14_3 preserved).

Stacked on matanl/bp-14703-strk-usd-oracle-alerts — merge that first.

Generated dev_grafana_alerts.json regenerated and verified identical (no-op).

[reviewable-StarkWare]

This change is 

[cursor]

PR Summary

Low Risk
Observability-only alert definitions; no runtime oracle or gas-pricing logic changes.

Overview
Adds frozen-feed monitoring for L1 gas price oracles alongside existing success/error alerts. eth_to_strk_rate_frozen and strk_to_usd_rate_frozen fire when sum(changes(<rate gauge>[1h])) stays below 1—i.e. the published rate did not move over an hour while the gauge is still present (no or vector(0), so missing metrics stay no-data).

Implementation introduces shared oracle_rate_frozen_alert on ETH_TO_STRK_RATE / SNIP35_STRK_USD_RATE, registers both in get_apollo_alerts, and mirrors them in dev_grafana_alerts.json with configurable severity placeholders. These alerts use ObserverApplicability::Applicable and omit the is_observer == 0 filter so observer nodes also surface a stale upstream price.

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