refactor(utils): create utils package and consolidate constants

Reorganized utility modules into structured package:
- average_utils.py → utils/average.py
- price_utils.py → utils/price.py
- Created utils/__init__.py with clean exports

Moved MINUTES_PER_INTERVAL to const.py (centralized constant
management), with re-exports in utils modules for backward
compatibility during migration.

Added comprehensive package docstring explaining scope:
- Pure data transformation functions (stateless)
- No HA entity/coordinator dependencies
- Clear separation from entity_utils/ (entity-specific logic)

Impact: Cleaner module structure, easier navigation. Follows
file organization policy from AGENTS.md (keep root clean).

custom_components/tibber_prices/utils/__init__.py

@@ -0,0 +1,63 @@
+"""
+Pure data transformation utilities for Tibber Prices integration.
+
+This package contains stateless, pure functions for data processing:
+- Time-window calculations (trailing/leading averages, min/max)
+- Price enrichment (differences, volatility, rating levels)
+- Statistical analysis (aggregation, trends)
+
+These functions operate on raw data structures (dicts, lists) and do NOT depend on:
+- Home Assistant entities or state management
+- Configuration entries or coordinators
+- Translation systems or UI-specific logic
+
+For entity-specific utilities (icons, colors, attributes), see entity_utils/ package.
+"""
+
+from __future__ import annotations
+
+from .average import (
+    calculate_current_leading_avg,
+    calculate_current_leading_max,
+    calculate_current_leading_min,
+    calculate_current_trailing_avg,
+    calculate_current_trailing_max,
+    calculate_current_trailing_min,
+    calculate_next_n_hours_avg,
+    round_to_nearest_quarter_hour,
+)
+from .price import (
+    aggregate_period_levels,
+    aggregate_period_ratings,
+    aggregate_price_levels,
+    aggregate_price_rating,
+    calculate_difference_percentage,
+    calculate_price_trend,
+    calculate_rating_level,
+    calculate_trailing_average_for_interval,
+    calculate_volatility_level,
+    enrich_price_info_with_differences,
+    find_price_data_for_interval,
+)
+
+__all__ = [
+    "aggregate_period_levels",
+    "aggregate_period_ratings",
+    "aggregate_price_levels",
+    "aggregate_price_rating",
+    "calculate_current_leading_avg",
+    "calculate_current_leading_max",
+    "calculate_current_leading_min",
+    "calculate_current_trailing_avg",
+    "calculate_current_trailing_max",
+    "calculate_current_trailing_min",
+    "calculate_difference_percentage",
+    "calculate_next_n_hours_avg",
+    "calculate_price_trend",
+    "calculate_rating_level",
+    "calculate_trailing_average_for_interval",
+    "calculate_volatility_level",
+    "enrich_price_info_with_differences",
+    "find_price_data_for_interval",
+    "round_to_nearest_quarter_hour",
+]

custom_components/tibber_prices/utils/price.py

@@ -7,10 +7,7 @@ import statistics
 from datetime import datetime, timedelta
 from typing import Any
 
-from homeassistant.util import dt as dt_util
-
-from .average_utils import round_to_nearest_quarter_hour
-from .const import (
+from custom_components.tibber_prices.const import (
     DEFAULT_VOLATILITY_THRESHOLD_HIGH,
     DEFAULT_VOLATILITY_THRESHOLD_MODERATE,
     DEFAULT_VOLATILITY_THRESHOLD_VERY_HIGH,
@@ -22,10 +19,12 @@ from .const import (
     VOLATILITY_MODERATE,
     VOLATILITY_VERY_HIGH,
 )
+from homeassistant.util import dt as dt_util
+
+from .average import round_to_nearest_quarter_hour
 
 _LOGGER = logging.getLogger(__name__)
 
-MINUTES_PER_INTERVAL = 15
 MIN_PRICES_FOR_VOLATILITY = 2  # Minimum number of price values needed for volatility calculation
 
 # Volatility factors for adaptive trend thresholds