feat(CUB-31): [Extrudex] Add filament usage tracking model

backend/Domain/Entities/FilamentUsage.cs

@@ -0,0 +1,73 @@
+using Extrudex.Domain.Base;
+
+namespace Extrudex.Domain.Entities;
+
+/// <summary>
+/// Tracks filament consumption for a specific print job on a specific spool.
+/// Each record captures the grams used, which printer consumed it, and when the
+/// usage was recorded. This enables granular per-job usage analytics, COGS
+/// reconciliation, and spool weight depletion tracking.
+///
+/// A single PrintJob may have multiple FilamentUsage records if multiple spools
+/// were consumed (e.g., multi-material prints via AMS).
+/// </summary>
+public class FilamentUsage : AuditableEntity
+{
+    /// <summary>
+    /// Foreign key to the print job that consumed this filament.
+    /// A usage record is always tied to a print job.
+    /// </summary>
+    public Guid PrintJobId { get; set; }
+
+    /// <summary>
+    /// Navigation to the print job that consumed this filament.
+    /// </summary>
+    public PrintJob PrintJob { get; set; } = null!;
+
+    /// <summary>
+    /// Foreign key to the spool (filament) that provided the material.
+    /// Links usage back to the specific physical spool for inventory tracking.
+    /// </summary>
+    public Guid SpoolId { get; set; }
+
+    /// <summary>
+    /// Navigation to the spool that provided the material.
+    /// </summary>
+    public Spool Spool { get; set; } = null!;
+
+    /// <summary>
+    /// Foreign key to the printer that executed the print job.
+    /// Denormalized from PrintJob for direct querying of per-printer usage.
+    /// </summary>
+    public Guid PrinterId { get; set; }
+
+    /// <summary>
+    /// Navigation to the printer that executed the print job.
+    /// </summary>
+    public Printer Printer { get; set; } = null!;
+
+    /// <summary>
+    /// Grams of filament consumed during this print job.
+    /// Derived from mm_extruded × cross_section_area × material_density,
+    /// or measured directly from AMS weight delta.
+    /// </summary>
+    public decimal GramsUsed { get; set; }
+
+    /// <summary>
+    /// Millimeters of filament extruded for this usage record.
+    /// The primary physical measurement; grams_used is derived from this.
+    /// </summary>
+    public decimal MmExtruded { get; set; }
+
+    /// <summary>
+    /// Timestamp when this usage record was created (UTC).
+    /// Represents when the usage was first logged, which may differ from
+    /// the print job's started_at or completed_at timestamps.
+    /// </summary>
+    public DateTime RecordedAt { get; set; } = DateTime.UtcNow;
+
+    /// <summary>
+    /// Optional notes about this usage record (e.g., "AMS tray 3", "manual weight check").
+    /// </summary>
+    public string? Notes { get; set; }
+}

backend/Domain/Entities/PrintJob.cs

@@ -97,4 +97,10 @@ public class PrintJob : AuditableEntity
     /// Optional notes about the print job (e.g., "First layer adhesion issues").
     /// </summary>
     public string? Notes { get; set; }
+
+    /// <summary>
+    /// Navigation collection of filament usage records for this print job.
+    /// Enables tracking granular per-spool consumption within a print.
+    /// </summary>
+    public ICollection<FilamentUsage> FilamentUsages { get; set; } = new List<FilamentUsage>();
 }

backend/Domain/Entities/Printer.cs

@@ -94,4 +94,10 @@ public class Printer : AuditableEntity
     /// Navigation collection of print jobs executed on this printer.
     /// </summary>
     public ICollection<PrintJob> PrintJobs { get; set; } = new List<PrintJob>();
+
+    /// <summary>
+    /// Navigation collection of filament usage records tracking consumption on this printer.
+    /// Enables querying per-printer filament usage and COGS.
+    /// </summary>
+    public ICollection<FilamentUsage> FilamentUsages { get; set; } = new List<FilamentUsage>();
 }

backend/Domain/Entities/Spool.cs

@@ -102,4 +102,10 @@ public class Spool : AuditableEntity
     /// Navigation collection of print jobs that consumed filament from this spool.
     /// </summary>
     public ICollection<PrintJob> PrintJobs { get; set; } = new List<PrintJob>();
+
+    /// <summary>
+    /// Navigation collection of filament usage records tracking consumption from this spool.
+    /// Enables querying how much filament was consumed per print job.
+    /// </summary>
+    public ICollection<FilamentUsage> FilamentUsages { get; set; } = new List<FilamentUsage>();
 }