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.Infrastructure/Data/ApplicationDbContext.cs

@@ -338,6 +338,11 @@ public class ApplicationDbContext : IdentityDbContext<ApplicationUser>, IDataPro
     /// <summary>Recurring transaction templates that auto-generate bills or expenses on a schedule; tenant-filtered with soft delete.</summary>
     public DbSet<RecurringTemplate> RecurringTemplates { get; set; }
 
+    /// <summary>Fixed assets subject to straight-line depreciation; tenant-filtered with soft delete.</summary>
+    public DbSet<FixedAsset> FixedAssets { get; set; }
+    /// <summary>One record per asset per period for each depreciation posting; soft-delete only.</summary>
+    public DbSet<FixedAssetDepreciationEntry> FixedAssetDepreciationEntries { get; set; }
+
     /// <summary>Credit notes received from vendors (returned goods, pricing disputes); tenant-filtered with soft delete.</summary>
     public DbSet<VendorCredit> VendorCredits { get; set; }
     /// <summary>Expense-reversal line items on a vendor credit; soft-delete only.</summary>
@@ -652,6 +657,36 @@ public class ApplicationDbContext : IdentityDbContext<ApplicationUser>, IDataPro
         modelBuilder.Entity<RecurringTemplate>().HasQueryFilter(e =>
             !e.IsDeleted && (IsPlatformAdmin || e.CompanyId == CurrentCompanyId));
 
+        // Fixed Assets: tenant-filtered with soft delete; depreciation entries soft-delete only
+        modelBuilder.Entity<FixedAsset>().HasQueryFilter(e =>
+            !e.IsDeleted && (IsPlatformAdmin || e.CompanyId == CurrentCompanyId));
+        modelBuilder.Entity<FixedAssetDepreciationEntry>().HasQueryFilter(e => !e.IsDeleted);
+
+        // FixedAsset → Account (three FKs): NoAction to avoid cascade conflicts; Account has no
+        // reverse collection for FixedAssets so WithMany() is anonymous for each.
+        modelBuilder.Entity<FixedAsset>()
+            .HasOne(fa => fa.AssetAccount)
+            .WithMany()
+            .HasForeignKey(fa => fa.AssetAccountId)
+            .OnDelete(DeleteBehavior.Restrict);
+        modelBuilder.Entity<FixedAsset>()
+            .HasOne(fa => fa.DepreciationExpenseAccount)
+            .WithMany()
+            .HasForeignKey(fa => fa.DepreciationExpenseAccountId)
+            .OnDelete(DeleteBehavior.Restrict);
+        modelBuilder.Entity<FixedAsset>()
+            .HasOne(fa => fa.AccumDepreciationAccount)
+            .WithMany()
+            .HasForeignKey(fa => fa.AccumDepreciationAccountId)
+            .OnDelete(DeleteBehavior.Restrict);
+
+        // FixedAssetDepreciationEntry → JournalEntry: NoAction (entries outlive their JE)
+        modelBuilder.Entity<FixedAssetDepreciationEntry>()
+            .HasOne(e => e.JournalEntry)
+            .WithMany()
+            .HasForeignKey(e => e.JournalEntryId)
+            .OnDelete(DeleteBehavior.NoAction);
+
         // Vendor Credits: tenant-filtered; child rows soft-delete only
         modelBuilder.Entity<VendorCredit>().HasQueryFilter(e =>
             !e.IsDeleted && (IsPlatformAdmin || e.CompanyId == CurrentCompanyId));