fix(data): harden FK pragma per-connection and seed concurrency

- Add SqliteForeignKeyInterceptor (DbConnectionInterceptor) registered via
  OnConfiguring so every IDbContextFactory-created context runs
  PRAGMA foreign_keys=ON, not only the MigrateAndConfigure context.
- DefaultListsSeeder: replace TOCTOU read-then-insert with atomic
  INSERT … SELECT … WHERE NOT EXISTS — one SQLite writer lock, no race.
- AppSettingsRepository.GetAsync: catch DbUpdateException on the
  get-or-create path and re-read so concurrent startup cannot throw.
- Migration 20260609000000_UniqueListName: de-duplicates empty list rows
  (startup-race leftovers) then adds a UNIQUE index on lists.name.
- ForeignKeyTests: verifies ON DELETE SET NULL (blocked_by_task_id) is
  enforced on a fresh DbContext with no manual PRAGMA call.

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

src/ClaudeDo.Data/Migrations/20260609000000_UniqueListName.cs

@@ -0,0 +1,39 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace ClaudeDo.Data.Migrations
+{
+    /// <inheritdoc />
+    public partial class UniqueListName : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            // Remove duplicate list rows that have no tasks — keep the oldest rowid.
+            // This handles the startup-race case where both App and Worker seeded
+            // the same default list names concurrently.
+            migrationBuilder.Sql("""
+                DELETE FROM lists
+                WHERE (SELECT COUNT(*) FROM tasks WHERE list_id = lists.id) = 0
+                  AND rowid NOT IN (
+                      SELECT MIN(l2.rowid) FROM lists l2 WHERE l2.name = lists.name
+                  )
+                """);
+
+            migrationBuilder.CreateIndex(
+                name: "idx_lists_name",
+                table: "lists",
+                column: "name",
+                unique: true);
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropIndex(
+                name: "idx_lists_name",
+                table: "lists");
+        }
+    }
+}