Add statistics data models for charts

[dmoo500]

What does this implement/fix?

Adds data models for the new Statistics Controller feature, defining the structure for playlog analytics responses (top items, distributions, time series, heatmaps, listening time).

⚠️ Note: While these models are correct, the backend statistics currently show incorrect play counts due to a playlog database design issue. See music-assistant/server#5700 for details.

Related issue (if applicable):

• History / Statistics controller backlog#21

New Models

• TopItemResult: Combines ItemMapping with play_count for top charts
• DistributionItem: Name/value pairs for pie/bar charts (artists, genres, decades)
• TimeSeriesPoint: Timestamp/value pairs for line charts (plays over time)
• HeatmapPoint: Hour/weekday/value tuples for activity heatmaps
• ListeningTimeItem: Name/minutes pairs for listening time charts

Types of changes

• New feature (non-breaking change which adds functionality) — new-feature

Checklist

• The code change is tested and works locally.
• pre-commit run --all-files passes.
• pytest passes, and tests have been added/updated under tests/ where applicable.
• Any consumer of a changed model is updated, and the companion PR in music-assistant/server is linked.
• I have read and complied with the project's AI Policy for any AI-assisted contributions.

Related PRs

• Server: Add Statistics Controller with playlog analytics server#5700
• Frontend: Add statistics and charts to discover page frontend#2526