Phase F: Add Invoice Write-Off, Fixed Assets, Period Locking, and 1099 Tracking

- Invoice Write-Off: WriteOff POST action in InvoicesController posts bad-debt JE
  (DR bad debt expense / CR AR), reduces customer balance, marks invoice WrittenOff;
  write-off modal added to Invoice Details view with expense account selector
- Fixed Assets: FixedAsset + FixedAssetDepreciationEntry entities with straight-line
  depreciation; FixedAssetsController (Index/Create/Edit/Details/PostDepreciation/Delete);
  PostDepreciation auto-generates one JE per asset per period, skips already-posted,
  fully-depreciated, and disposed assets; full CRUD views + nav link
- Period Locking: Company.BookLockedThrough field; AccountingPeriodValidator static helper;
  lock check added to JE Post and Bill Create (blocks backdating into closed periods);
  SetPeriodLock action + date picker UI in Company Settings Accounting section
- 1099 Tracking: Is1099Vendor flag on Vendor entity + DTOs; checkbox in Create/Edit views;
  TaxReporting1099 report action + view lists payments by year, flags vendors >= $600;
  report card added to Reports Landing
- Migration AddFixedAssetsLockAnd1099 applied

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/PowderCoating.Core/Entities/Accounting.cs

@@ -334,3 +334,64 @@ public class TaxRate : BaseEntity
     public bool IsDefault { get; set; }
     public bool IsActive { get; set; } = true;
 }
+
+/// <summary>
+/// A depreciable fixed asset (oven, blast cabinet, spray booth, vehicle, etc.).
+/// Stores straight-line depreciation parameters and links to the three GL accounts needed
+/// to auto-post monthly depreciation journal entries.
+/// </summary>
+public class FixedAsset : BaseEntity
+{
+    public string Name { get; set; } = string.Empty;
+    public string? Description { get; set; }
+    public DateTime PurchaseDate { get; set; }
+    public decimal PurchaseCost { get; set; }
+    /// <summary>Residual value at end of useful life (often $0 for shop equipment).</summary>
+    public decimal SalvageValue { get; set; } = 0;
+    /// <summary>Total depreciation period in months (e.g., 60 = 5 years).</summary>
+    public int UsefulLifeMonths { get; set; }
+    /// <summary>Running total of depreciation posted so far.</summary>
+    public decimal AccumulatedDepreciation { get; set; } = 0;
+    public bool IsDisposed { get; set; } = false;
+    public DateTime? DisposalDate { get; set; }
+
+    // Computed — not persisted
+    /// <summary>Current net book value: PurchaseCost minus AccumulatedDepreciation.</summary>
+    public decimal BookValue => PurchaseCost - AccumulatedDepreciation;
+    /// <summary>Straight-line monthly depreciation amount.</summary>
+    public decimal MonthlyDepreciation => UsefulLifeMonths > 0
+        ? Math.Round((PurchaseCost - SalvageValue) / UsefulLifeMonths, 2) : 0;
+
+    // GL account links — all optional; assets without accounts can be tracked but not auto-posted
+    /// <summary>Balance Sheet FixedAsset account (debited when asset is purchased).</summary>
+    public int? AssetAccountId { get; set; }
+    /// <summary>P&L Depreciation Expense account (debited each period).</summary>
+    public int? DepreciationExpenseAccountId { get; set; }
+    /// <summary>Balance Sheet Accumulated Depreciation account (credited each period).</summary>
+    public int? AccumDepreciationAccountId { get; set; }
+
+    // Navigation
+    public virtual Account? AssetAccount { get; set; }
+    public virtual Account? DepreciationExpenseAccount { get; set; }
+    public virtual Account? AccumDepreciationAccount { get; set; }
+    public virtual ICollection<FixedAssetDepreciationEntry> DepreciationEntries { get; set; } = new List<FixedAssetDepreciationEntry>();
+}
+
+/// <summary>
+/// Records each periodic depreciation posting for a fixed asset. One record per asset per
+/// month/year combination; linked to the JournalEntry that was created so the posting
+/// can be traced back through the GL.
+/// </summary>
+public class FixedAssetDepreciationEntry : BaseEntity
+{
+    public int FixedAssetId { get; set; }
+    public int PeriodYear { get; set; }
+    public int PeriodMonth { get; set; }
+    public decimal Amount { get; set; }
+    /// <summary>The JE that was posted for this depreciation period (null if manually recorded).</summary>
+    public int? JournalEntryId { get; set; }
+
+    // Navigation
+    public virtual FixedAsset FixedAsset { get; set; } = null!;
+    public virtual JournalEntry? JournalEntry { get; set; }
+}

src/PowderCoating.Core/Entities/Company.cs

@@ -112,6 +112,12 @@ public class Company : BaseEntity
     /// </summary>
     public AccountingMethod AccountingMethod { get; set; } = AccountingMethod.Accrual;
 
+    /// <summary>
+    /// When set, prevents creating or editing accounting entries (JEs, bills, expenses) with dates
+    /// on or before this date. Protects closed periods from accidental backdating. Null = no lock.
+    /// </summary>
+    public DateTime? BookLockedThrough { get; set; }
+
     // Settings
     public string? TimeZone { get; set; } = "America/New_York";
     public byte[]? LogoData { get; set; }  // Legacy - kept for backward compatibility

src/PowderCoating.Core/Entities/SupportingEntities.cs

@@ -35,6 +35,10 @@ public class Vendor : BaseEntity
     /// <summary>Default expense account pre-filled on new bill line items for this vendor.</summary>
     public int? DefaultExpenseAccountId { get; set; }
 
+    // 1099 Contractor tracking
+    /// <summary>When true, this vendor is an independent contractor subject to 1099-NEC reporting.</summary>
+    public bool Is1099Vendor { get; set; } = false;
+
     // Navigation
     public virtual ICollection<InventoryItem> InventoryItems { get; set; } = new List<InventoryItem>();
     public virtual ICollection<Bill> Bills { get; set; } = new List<Bill>();