mvp phase ε.2: register prometheus_remote engine in EngineRouter

[zzylol]

Summary

• Adds PrometheusForwardEngine (HTTP-forwarder to Prometheus's standard /api/v1/query) and registers it on the capability router under data_source_id = prometheus_remote when ASAP_PROMETHEUS_QUERY_URL is set. Wired into both src/main.rs and src/bin/precompute_engine.rs. Mirror of Step-2.3's ThanosForwardEngine (PR mvp step 2.3: backend HTTP-forward to thanos-query for Path A2 archive #97), pointed at Prometheus instead of a thanos-query sidecar.
• Adds StorageBackend::PrometheusRemote (data_source_id = "prometheus_remote") to asap_types::capability_matching so compatible_storage_backends returns [PrometheusRemote] for Mode 3 metrics (single backend, no failover — the metric's data never landed in ASAP-managed storage). parse_engine_string in BackendStorageRouting accepts the controller's engine: prometheus_remote string.
• Fail-loud contract: env unset → engine not registered → routing entry referencing prometheus_remote returns NoEngineRegistered. Engine reachable but Prometheus down → Backend(prometheus_unreachable: ...) → 503 with data_source_quirk: prometheus_unreachable in the response infos.

Context

Phase ε.1 (controller, separate repo) emits engine: prometheus_remote in the BackendStorageRouting JSON for metrics whose raw data is shipped to Prometheus's native OTLP receiver (controller's RawAtEdgePrometheusArchive mode). Phase ε.2 (this PR) closes the loop by registering an engine that handles those routing entries. Existing thanos_archive / gorilla_archive registrations are unchanged.

Test plan

• cargo build --release — green
• cargo test --release --lib — 33 pre-existing backend failures unchanged; +17 new tests (12 in engines::prometheus::forward::tests, 3 in routing::backend_storage_routing::tests, 2 in routing::engine_router::tests) all pass
• HTTP forward path (mock Prometheus axum server)
• Unreachable upstream → 503 with quirk infos
• Env-set vs env-unset registration contract
• Routing-table JSON engine: prometheus_remote parses to StorageBackend::PrometheusRemote
• Router dispatches StorageBackend::PrometheusRemote to the registered engine; missing-engine surfaces NoEngineRegistered

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