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/ClaudeDoDbContext.cs

@@ -1,6 +1,8 @@
+using System.Data.Common;
 using ClaudeDo.Data.Models;
 using ClaudeDo.Data.Seeding;
 using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore.Diagnostics;
 using Microsoft.EntityFrameworkCore.Infrastructure;
 using Microsoft.EntityFrameworkCore.Storage;
 using Microsoft.EntityFrameworkCore.Storage.ValueConversion;
@@ -9,8 +11,35 @@ namespace ClaudeDo.Data;
 
 public class ClaudeDoDbContext : DbContext
 {
+    // Runs PRAGMA foreign_keys=ON on every EF-managed connection open so FK
+    // enforcement is active for all IDbContextFactory-created contexts, not
+    // just the single context used in MigrateAndConfigure.
+    private sealed class SqliteForeignKeyInterceptor : DbConnectionInterceptor
+    {
+        internal static readonly SqliteForeignKeyInterceptor Instance = new();
+
+        public override void ConnectionOpened(DbConnection connection, ConnectionEndEventData eventData)
+            => Apply(connection);
+
+        public override Task ConnectionOpenedAsync(DbConnection connection, ConnectionEndEventData eventData, CancellationToken cancellationToken = default)
+        {
+            Apply(connection);
+            return Task.CompletedTask;
+        }
+
+        private static void Apply(DbConnection connection)
+        {
+            using var cmd = connection.CreateCommand();
+            cmd.CommandText = "PRAGMA foreign_keys=ON;";
+            cmd.ExecuteNonQuery();
+        }
+    }
+
     public ClaudeDoDbContext(DbContextOptions<ClaudeDoDbContext> options) : base(options) { }
 
+    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
+        => optionsBuilder.AddInterceptors(SqliteForeignKeyInterceptor.Instance);
+
     public DbSet<TaskEntity> Tasks => Set<TaskEntity>();
     public DbSet<ListEntity> Lists => Set<ListEntity>();
     public DbSet<ListConfigEntity> ListConfigs => Set<ListConfigEntity>();