feat(optimization): support a 15-minute optimization interval

The genetic optimizer was hard-wired to an hourly grid and forced
optimization.interval to 3600 s. Generalize it to a configurable slot grid
of length prediction.hours * (3600 / interval), accepting 900 (15 min) in
addition to the default 3600 (1 hour) so the optimizer can schedule on a
quarter-hour grid for 15-minute dynamic electricity tariffs.

- genetic.py: slot_duration_h / slots_per_hour / total_slots helpers; all GA
  vectors sized by total_slots; simulate()/evaluate() indexed by start slot.
- geneticparams.py: allow {900, 3600}; scale the load power series to per-slot
  energy, mirroring the PV series.
- battery.py / inverter.py: scale power caps to per-slot energy caps via
  slot_duration_h; homeappliance.py carries the hook.
- geneticsolution.py: serialize solution and plan on the slot grid (interval
  freq, start-slot offset, second-based instruction instants).

The default 3600 s interval keeps the previous hourly behaviour; the genetic
regression suite is unchanged. Adds tests for the 15-minute slot grid.

src/akkudoktoreos/optimization/genetic/geneticparams.py

@@ -194,9 +194,17 @@ class GeneticOptimizationParameters(
         if cls.config.optimization.interval is None:
             logger.info("Optimization interval unknown - defaulting to 3600 seconds.")
             cls.config.optimization.interval = 3600
-        if cls.config.optimization.interval != 3600:
-            logger.info(
-                "Optimization interval '{}' seconds not supported - forced to 3600 seconds."
+        # The genetic optimizer runs on a fixed slot grid whose length is
+        # prediction.hours * (3600 / interval). 900 s (15 min) enables a
+        # quarter-hour grid for 15-minute electricity tariffs; the default
+        # 3600 s keeps the established hourly resolution. Other values fall back
+        # to 3600 s.
+        allowed_intervals = (3600, 900)
+        if cls.config.optimization.interval not in allowed_intervals:
+            logger.warning(
+                "Optimization interval {} seconds not in {} - forcing 3600 seconds.",
+                cls.config.optimization.interval,
+                allowed_intervals,
             )
             cls.config.optimization.interval = 3600
         # Check genetic algorithm definitions
@@ -306,12 +314,18 @@ class GeneticOptimizationParameters(
                 # Retry
                 continue
             try:
-                loadforecast_power_w = cls.prediction.key_to_array(
-                    key="loadforecast_power_w",
-                    start_datetime=parameter_start_datetime,
-                    end_datetime=parameter_end_datetime,
-                    interval=interval,
-                    fill_method="ffill",
+                # Load is a power series [W] that the genetic optimizer consumes
+                # as Wh-per-slot. Scale by interval/3600 (mirrors the PV forecast
+                # above) so a 15-min slot sees a quarter of the hourly energy.
+                loadforecast_power_w = (
+                    cls.prediction.key_to_array(
+                        key="loadforecast_power_w",
+                        start_datetime=parameter_start_datetime,
+                        end_datetime=parameter_end_datetime,
+                        interval=interval,
+                        fill_method="ffill",
+                    )
+                    * power_to_energy_per_interval_factor
                 ).tolist()
             except:
                 logger.info(