fix(http): consult per-metric BackendStorageRouting on every query (issue #46 ⑤)

[zzylol]

Summary

Closes the issue-46 v2 criterion ⑤ partial-pass: data_source: gorilla_archive never landed on cold-archive responses because the HTTP query handler bypassed EngineRouter for every query.

Root cause (per the v2 root-cause analysis on issue #46): the Phase-5 dispatch read the per-metric StorageBackend from StreamingConfig::storage_backend() — a single field. The deployed precompute_engine binary loads the streaming config via StreamingConfig::from_yaml_file → from_yaml_data, which constructs via Self::new(...) and always defaults the axis to SketchWarmTier. So the EngineRouter was effectively never consulted in production.

Per the user's architecture correction: the streaming engine never sees Gorilla data on its OTLP-ingest path (the agent's gorillas3processor writes chunks directly to S3), so there is nothing for StreamingConfig::from_yaml_data to learn. The fix lives in a separate per-metric routing layer.

What changed

• New BackendStorageRouting data type — a {metric_name: StorageBackend} map loaded from YAML at startup. Schema:

  default: sketch_warm_tier
  metrics:
    http_requests_total: gorilla_s3_archive

• New --backend-storage-routing CLI flag (alias ASAP_BACKEND_STORAGE_ROUTING env var) on both the precompute_engine binary (used by the deployed Docker image) and the legacy query_engine_rust binary.
• HttpServer::with_backend_storage_routing(...) wires the routing table into AppState.
• process_query_request now consults the routing table on every PromQL query: parses the query, extracts the first metric name from the AST via promql_parser, looks up the storage backend, and dispatches through EngineRouter for any per-metric override. Falls back to the streaming-config single axis when no routing table is wired (preserves pre-Phase-5 behaviour).

Test plan

• 6 new BackendStorageRouting unit tests (empty router, YAML round-trip, defaults).
• 3 new end-to-end production-path unit tests in http.rs::tests:
  • http_production_path_routes_archive_metric_via_routing_table — streaming-config stays default (SketchWarmTier); routing table flips http_requests_total → gorilla_s3_archive; the response carries data_source: gorilla_archive and the mock GorillaQueryEngine is hit exactly once.
  • http_production_path_unlisted_metric_falls_back_to_warm_tier — non-listed metric still hits SimpleEngine direct path.
  • http_production_path_default_axis_routes_all_metrics — default: gorilla_s3_archive (no per-metric overrides) routes everything.
• All 5 pre-existing http-router tests still pass (http_routes_archive_metric_to_gorilla_engine, etc.).
• cargo test -p query_engine_rust --lib drivers::query::servers::http: 25 tests pass.
• cargo build -p query_engine_rust --bins: clean.

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