feat(elecprice): serve native 15-minute Tibber prices for the quarter-hour grid

Request QUARTER_HOURLY exchange prices from Tibber and store them at their
native resolution instead of pre-averaging to hourly values. EOS resamples the
stored records onto the optimization grid on demand (key_to_array), so keeping
the native step size lets both the hourly (interval=3600) and the 15-minute
(interval=900) optimizer be fed the correct grid automatically.

- GraphQL: priceInfoRange resolution HOURLY -> QUARTER_HOURLY (last 960).
- _hourly_series -> _normalize_series: dedupe by timestamp (mean) + sort, no
  1h aggregation; add _resolution_seconds (median of timestamp diffs, fallback
  3600s).
- Resolution-agnostic ETS extrapolation: seasonal windows and history
  thresholds are scaled by slots_per_hour, needed forecast length and the
  prediction index step are computed in slots. Hourly behaviour is unchanged
  (slots_per_hour=1 -> 168/24 seasonal periods, hourly steps).
- Tests: replace the 1h-averaging test with resolution-preserving + dedup
  tests, assert QUARTER_HOURLY in the query, add a 15-min end-to-end test
  (native storage stays 15min, slot-based seasonal periods = 96, 15-min
  forecast index). Hourly backward-compat tests stay green unchanged.

CHANGELOG.md

@@ -20,6 +20,11 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
   default 3600 s interval keeps the previous hourly behaviour. The new sub-hourly PV
   providers (pvnode, Forecast.Solar, Solcast) feed their native resolution straight
   into the quarter-hour grid.
+  - The Tibber electricity price provider now requests native 15-minute exchange prices
+    (`priceInfoRange(resolution: QUARTER_HOURLY)`) and stores them at their native
+    resolution, so both the hourly and the 15-minute optimizer are fed the correct
+    grid. The seasonal price extrapolation is resolution-agnostic and stays identical
+    at the default hourly resolution.
 
 ## 0.3.0 (2026-03-17)