M1: audit log (Governance) + edit-distance metric

SharedKernel:
- IAuditLog/AuditEvent — append-only audit contract any module writes through.
- EditDistance (Levenshtein + normalized) — the north-star metric, available from day
  one; consumed at edit-and-approve in M5.

Governance module (references SharedKernel only):
- AuditEntry entity; internal GovernanceDbContext (schema "governance") +
  InitialGovernance migration; AuditLog implements IAuditLog.
- GET /api/governance/audit — owner-only (ViewAuditLog), returns recent entries.

Wiring (via the SharedKernel IAuditLog interface — no module references Governance):
- OrgBoard records team.created, task.created, task.moved, task.assigned.
- Identity records invitation.created, member.joined.

Verified: build green; ArchitectureTests 8/8 (Governance references only SharedKernel;
audit flows through the shared interface); IntegrationTests 20/20 — board flow now
asserts task.created/task.moved appear in the audit log, plus EditDistance unit tests.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/TeamUp.IntegrationTests/EditDistanceTests.cs

@@ -0,0 +1,26 @@
+using TeamUp.SharedKernel.Metrics;
+using Xunit;
+
+namespace TeamUp.IntegrationTests;
+
+/// <summary>Unit coverage for the north-star metric helper (no database).</summary>
+public sealed class EditDistanceTests
+{
+    [Theory]
+    [InlineData("", "", 0)]
+    [InlineData("abc", "abc", 0)]
+    [InlineData("", "abc", 3)]
+    [InlineData("abc", "", 3)]
+    [InlineData("kitten", "sitting", 3)]
+    [InlineData("spec v1", "spec v2", 1)]
+    public void Levenshtein_counts_edits(string a, string b, int expected) =>
+        Assert.Equal(expected, EditDistance.Levenshtein(a, b));
+
+    [Fact]
+    public void Normalized_is_zero_when_unchanged() =>
+        Assert.Equal(0d, EditDistance.Normalized("approved as-is", "approved as-is"));
+
+    [Fact]
+    public void Normalized_is_between_zero_and_one() =>
+        Assert.InRange(EditDistance.Normalized("the AI wrote this", "the human edited this"), 0d, 1d);
+}