Move CRDT commits to a composite (ProjectId, Id) PK + add unique index on Id

Background

Split out of the template-based project-creation work (branch claude/investigate-issue-1920-f6yHG, PR #2281). That PR pivoted to importing a JSON project snapshot and no longer needs this server-side schema change, so it's being deferred to its own change rather than riding along as unrelated scope.

What this is

Changes the server-side CrdtCommits primary key from (Id) to a composite (ProjectId, Id), and adjusts CommitEntityConfiguration, CrdtCommitService, and the tests to match.

Also wanted: a UNIQUE index on Id

We still never expect commit Ids to overlap — especially now that Harmony no longer lets a caller specify a commit's Id (they're minted internally). A composite (ProjectId, Id) PK on its own would permit the same Id under two different ProjectIds. Add a unique index on Id alongside the composite PK so global commit-Id uniqueness is actually enforced.

Patch

This is the non-generated part of the change only. The EF model snapshots — *.Designer.cs and LexBoxDbContextModelSnapshot.cs — are regenerated by dotnet ef migrations add, so they're omitted here; regenerate them against develop when re-adding the migration.

diff --git a/backend/LexBoxApi/Services/CrdtCommitService.cs b/backend/LexBoxApi/Services/CrdtCommitService.cs
index d4ec51f8c..6fa4768cb 100644
--- a/backend/LexBoxApi/Services/CrdtCommitService.cs
+++ b/backend/LexBoxApi/Services/CrdtCommitService.cs
@@ -16,7 +16,10 @@ public class CrdtCommitService(LexBoxDbContext dbContext)
         await using var transaction = await dbContext.Database.BeginTransactionAsync(token);
         var linqToDbContext = dbContext.CreateLinqToDBContext();
         await using var tmpTable = await linqToDbContext.CreateTempTableAsync<ServerCommit>($"tmp_crdt_commit_import_{projectId}__{Guid.NewGuid()}", cancellationToken: token);
-        //Stamp ProjectId while streaming so the merge below can be a plain column-to-column copy.
+        //Stamp ProjectId while streaming so the merge below can be a plain column-to-column copy
+        //AND so OnTargetKey (composite PK (ProjectId, Id)) matches against the right tenant —
+        //client-supplied ProjectId on the wire is untrusted; the URL path's projectId already
+        //cleared permissionService.
         //A projection lambda here would let linq2db v6 wrap our Sql.Expr<...>::jsonb cast in the
         //EF value-converter (JsonSerializer.Serialize) and fail SQL translation.
         var stampedCommits = commits.Select(c => { c.ProjectId = projectId; return c; });
diff --git a/backend/LexData/Entities/CommitEntityConfiguration.cs b/backend/LexData/Entities/CommitEntityConfiguration.cs
index 99c7ee188..550787312 100644
--- a/backend/LexData/Entities/CommitEntityConfiguration.cs
+++ b/backend/LexData/Entities/CommitEntityConfiguration.cs
@@ -18,7 +18,12 @@ public class CommitEntityConfiguration : IEntityTypeConfiguration<ServerCommit>
     public void Configure(EntityTypeBuilder<ServerCommit> builder)
     {
         builder.ToTable("CrdtCommits");
-        builder.HasKey(c => c.Id);
+        // PK is (ProjectId, Id): Commit.Id alone is not unique across projects (FwLite clients
+        // can mint the same Id for two different projects — e.g. a shared seed-commit pattern
+        // that ignores ProjectId), and the old Id-only PK caused the second arriving copy to be
+        // silently dropped by CrdtCommitService's MERGE ON TARGET KEY. Scoping the key by
+        // ProjectId lets both projects keep their own row.
+        builder.HasKey(c => new { c.ProjectId, c.Id });
         builder.ComplexProperty(c => c.HybridDateTime);
         builder.HasOne<Project>().WithMany()
             .HasPrincipalKey(project => project.Id)
diff --git a/backend/LexData/Migrations/20260528184900_ChangeCrdtCommitsPkToCompositeProjectIdId.cs b/backend/LexData/Migrations/20260528184900_ChangeCrdtCommitsPkToCompositeProjectIdId.cs
new file mode 100644
index 000000000..def1be60a
--- /dev/null
+++ b/backend/LexData/Migrations/20260528184900_ChangeCrdtCommitsPkToCompositeProjectIdId.cs
@@ -0,0 +1,45 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace LexData.Migrations
+{
+    /// <inheritdoc />
+    public partial class ChangeCrdtCommitsPkToCompositeProjectIdId : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropPrimaryKey(
+                name: "PK_CrdtCommits",
+                table: "CrdtCommits");
+
+            migrationBuilder.DropIndex(
+                name: "IX_CrdtCommits_ProjectId",
+                table: "CrdtCommits");
+
+            migrationBuilder.AddPrimaryKey(
+                name: "PK_CrdtCommits",
+                table: "CrdtCommits",
+                columns: new[] { "ProjectId", "Id" });
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropPrimaryKey(
+                name: "PK_CrdtCommits",
+                table: "CrdtCommits");
+
+            migrationBuilder.AddPrimaryKey(
+                name: "PK_CrdtCommits",
+                table: "CrdtCommits",
+                column: "Id");
+
+            migrationBuilder.CreateIndex(
+                name: "IX_CrdtCommits_ProjectId",
+                table: "CrdtCommits",
+                column: "ProjectId");
+        }
+    }
+}
diff --git a/backend/Testing/LexCore/Services/CrdtCommitServiceTests.cs b/backend/Testing/LexCore/Services/CrdtCommitServiceTests.cs
index cfd853c6c..ee192a328 100644
--- a/backend/Testing/LexCore/Services/CrdtCommitServiceTests.cs
+++ b/backend/Testing/LexCore/Services/CrdtCommitServiceTests.cs
@@ -192,6 +192,27 @@ public class CrdtCommitServiceTests
         _lexBoxDbContext.CrdtCommits(commit.ProjectId).Should().HaveCountGreaterThan(0);
     }
 
+    [Fact]
+    public async Task SameCommitIdInTwoDifferentProjectsKeepsBoth()
+    {
+        // Regression: with an Id-only PK the second project's identical Commit.Id was silently dropped by the MERGE's OnTargetKey; composite (ProjectId, Id) keeps both.
+        var projects = await _lexBoxDbContext.Projects.Select(p => p.Id).Take(2).ToArrayAsync();
+        if (projects.Length < 2)
+            throw new InvalidOperationException("Test requires at least 2 seeded projects.");
+
+        var sharedCommitId = Guid.NewGuid();
+        var commitForA = CreateCommit(Guid.NewGuid(), Guid.NewGuid(), DateTime.UtcNow, sharedCommitId);
+        var commitForB = CreateCommit(Guid.NewGuid(), Guid.NewGuid(), DateTime.UtcNow, sharedCommitId);
+
+        await _crdtCommitService.AddCommits(projects[0], AsAsync([commitForA]));
+        await _crdtCommitService.AddCommits(projects[1], AsAsync([commitForB]));
+
+        var inA = await _lexBoxDbContext.CrdtCommits(projects[0]).Where(c => c.Id == sharedCommitId).ToArrayAsync();
+        var inB = await _lexBoxDbContext.CrdtCommits(projects[1]).Where(c => c.Id == sharedCommitId).ToArrayAsync();
+        inA.Should().ContainSingle().Which.ClientId.Should().Be(commitForA.ClientId);
+        inB.Should().ContainSingle().Which.ClientId.Should().Be(commitForB.ClientId);
+    }
+
 
     private async Task<ServerCommit> AddTestCommit()
     {

Review backend/LexData/Entities/CommitEntityConfiguration.cs, backend/LexBoxApi/Services/CrdtCommitService.cs, the named migration, and CrdtCommitServiceTests.cs. Start with the existing composite-key regression test and migration, then regenerate the EF snapshots with dotnet ef migrations add; done means both projects retain the shared commit ID, uniqueness is enforced, and the tests pass.