Fix StatisticsService dependency injection lifetime mismatch

[Copilot]

StatisticsService was registered as Singleton while depending on Transient-scoped repositories (IWordRepository, IUserRepository), causing the service to capture and retain the first instances indefinitely instead of getting fresh instances per scope.

Changes

• Changed StatisticsService registration from AddSingleton to AddScoped in Backend/Startup.cs

// Before
services.AddSingleton<IStatisticsService, StatisticsService>();

// After  
services.AddScoped<IStatisticsService, StatisticsService>();

Scoped lifetime aligns with ASP.NET Core conventions for services with database dependencies and ensures proper disposal within HTTP request boundaries.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Fix StatisticsService Lifetime Registration</issue_title>
<issue_description>Update StatisticsService registration from Singleton to Scoped lifetime to fix
dependency injection mismatch.

In Backend/Startup.cs:

• Change the StatisticsService registration from AddSingleton to AddScoped
• No changes needed to StatisticsService.cs implementation</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Fix StatisticsService Lifetime Registration #4088

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This change is 

Summary by CodeRabbit

• Chores
  • Optimized internal service lifecycle management for improved performance and resource efficiency.

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📝 Walkthrough

Walkthrough

The dependency injection container configuration is modified to register StatisticsService with Scoped lifetime instead of Singleton. This ensures a new instance is created per request scope rather than reused globally throughout the application.

Changes

Cohort / File(s)	Summary
Dependency Injection Lifetime Configuration Backend/Startup.cs	Changed IStatisticsService registration from AddSingleton to AddScoped to align dependency injection lifetime with the service's actual usage requirements.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

Poem

A rabbit hops through code so neat, 🐰
One tiny change makes it complete!
Singleton to Scoped, a lifetime swap,
Fresh instances at every hop!
No singletons hoarding resources here,
Just cleaner scopes, fresh and clear! ✨

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately describes the main change: updating StatisticsService's DI lifetime from Singleton to Scoped to fix a dependency injection mismatch.
Linked Issues check	✅ Passed	The PR fully addresses the linked issue #4088 by changing StatisticsService registration from AddSingleton to AddScoped in Backend/Startup.cs as required.
Out of Scope Changes check	✅ Passed	The PR contains only the required change to Backend/Startup.cs with no out-of-scope modifications beyond the dependency injection lifetime update.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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✨ Finishing touches

• 📝 Generate docstrings

---

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📥 Commits

Reviewing files that changed from the base of the PR and between d2f2aa3 and bc03594.

📒 Files selected for processing (1)

• Backend/Startup.cs

🧰 Additional context used

🧬 Code graph analysis (1)

Backend/Startup.cs (1)

Backend/Services/StatisticsService.cs (2)

• StatisticsService (12-370)
• StatisticsService (20-26)

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🔇 Additional comments (1)

Backend/Startup.cs (1)

277-277: Correct fix for the dependency injection lifetime mismatch.

Changing StatisticsService from Singleton to Scoped resolves the issue where a Singleton service was capturing Transient-scoped repository instances (IWordRepository, IUserRepository). With Scoped lifetime:

• A fresh instance is created per HTTP request
• Repository dependencies are properly instantiated and disposed within request scope
• Aligns with ASP.NET Core conventions for services with data access dependencies

The service implementation is stateless with a lightweight constructor, so there are no performance or thread-safety concerns from this change.

Note: While reviewing the DI configuration, MergeService (line 262) appears to have the same lifetime mismatch pattern—it's registered as Singleton but depends on Transient repositories. Consider addressing that in a follow-up if not already tracked.

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[imnasnainaec]

@coderabbitai review

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[Copilot]

Pull request overview

This PR fixes a dependency injection lifetime mismatch where StatisticsService was registered as a Singleton while depending on Transient-scoped repositories. This caused the service to capture and retain the first repository instances indefinitely, leading to potential issues with stale database connections and incorrect data isolation between requests.

Key Changes:

• Changed StatisticsService registration from AddSingleton to AddScoped in the DI container

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.59%. Comparing base (d2f2aa3) to head (bc03594).

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #4089       +/-   ##
===========================================
+ Coverage   74.57%   85.59%   +11.02%     
===========================================
  Files         295       54      -241     
  Lines       10938     4777     -6161     
  Branches     1372      589      -783     
===========================================
- Hits         8157     4089     -4068     
+ Misses       2385      544     -1841     
+ Partials      396      144      -252     

Flag	Coverage Δ
backend	85.59% <ø> (ø)
frontend	?

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[imnasnainaec]

@imnasnainaec reviewed all commit messages and made 1 comment.
Reviewable status: 0 of 1 files reviewed, 1 unresolved discussion.

---

Backend/Startup.cs line 277 at r1 (raw file):

            // Statistics types
            services.AddScoped<IStatisticsService, StatisticsService>();

Updated issue: this should be AddTransient instead of AddScoped.

[imnasnainaec]

Replaced by #4093