fix(backend): Register IApplicationDbContext interface in DI container - BUG-006

Fixed critical P0 bug where application failed to start due to missing
IApplicationDbContext registration in dependency injection container.

Root Cause:
- Sprint command handlers (CreateSprint, UpdateSprint, etc.) depend on IApplicationDbContext
- PMDbContext implements IApplicationDbContext but interface was not registered in DI
- ASP.NET Core DI validation failed at application startup

Solution:
- Added IApplicationDbContext interface registration in ModuleExtensions.cs
- Maps interface to PMDbContext implementation using service provider

Impact:
- Application can now start successfully
- All Sprint command handlers can resolve their dependencies
- Docker container startup will succeed

Testing:
- Local build: SUCCESS
- Docker build: PENDING QA validation

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

colaflow-api/src/ColaFlow.API/Extensions/ModuleExtensions.cs

@@ -46,6 +46,10 @@ public static class ModuleExtensions
             });
         }
 
+        // Register IApplicationDbContext interface (required by command handlers)
+        services.AddScoped<ColaFlow.Modules.ProjectManagement.Application.Common.Interfaces.IApplicationDbContext>(
+            sp => sp.GetRequiredService<PMDbContext>());
+
         // Register HTTP Context Accessor (for tenant context)
         services.AddHttpContextAccessor();