Statistics data layer

[MessiasLima]

Description

Create the data layer for task statistics. This includes defining the statistics domain model and a repository that computes statistics from the task data stored in Room.

Free statistics include:

• Total tasks created
• Total tasks completed
• Current streak: number of consecutive days with at least one task completed
• Longest streak: the longest consecutive-day completion streak ever achieved

Premium statistics (gated behind #130) include:

• Completion rate (percentage of tasks completed)
• Average tasks completed per day over the last 7 days
• Most productive day of the week
• Tasks completed by priority breakdown

The StatisticsRepository exposes these as Flow<Statistics> so that UI screens can observe them reactively.

Depends on #120 (Task domain model and data layer), #130 (Premium gating infrastructure).

Scope

In scope: Statistics data class, StatisticsRepository interface and implementation, Room queries to compute statistics, free vs. premium statistics separation, and unit tests.

Out of scope: statistics UI screen (separate ticket #26).

Acceptance criteria

• GIVEN the StatisticsRepository is initialized
  WHEN the free statistics are collected
  THEN they include total tasks created, total tasks completed, current streak, and longest streak
• GIVEN the StatisticsRepository is initialized
  WHEN the premium statistics are collected
  THEN they include completion rate, average tasks completed per day (last 7 days), most productive day of the week, and tasks completed by priority breakdown
• GIVEN a user completes a task today
  AND they completed at least one task yesterday
  WHEN the current streak is computed
  THEN the streak increments by 1
• GIVEN a user completes a task today
  AND they did not complete any task yesterday
  WHEN the current streak is computed
  THEN the streak resets to 1
• GIVEN the current streak is longer than the previously stored longest streak
  WHEN the statistics are computed
  THEN the longest streak is updated
• GIVEN the user is not a premium subscriber
  WHEN free statistics are requested
  THEN all free statistics are returned
• GIVEN the user is a premium subscriber
  WHEN statistics are requested
  THEN both free and premium statistics are returned
• GIVEN the StatisticsRepository is injected via Koin
  WHEN a ViewModel needs statistics
  THEN it can access them via StatisticsRepository.getStatistics(): Flow<Statistics>