v1: Timeseries matrix endpoints with pre-warmed cache

[braddf]

GET /forecasts/timeseries?region_type=… and
GET /generation/timeseries?region_type=…;
one cache key per region UUID;
in-memory sub-queries for start_utc/end_utc and region_ids filters;
503 + Retry-After on cold cache.

[braddf]

TBC naming of these routes