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/Seeding/DefaultListsSeeder.cs

@@ -1,4 +1,3 @@
-using ClaudeDo.Data.Models;
 using Microsoft.EntityFrameworkCore;
 
 namespace ClaudeDo.Data.Seeding;
@@ -9,17 +8,18 @@ public static class DefaultListsSeeder
 
     public static async Task SeedAsync(ClaudeDoDbContext ctx, CancellationToken ct = default)
     {
-        var existing = await ctx.Lists.Select(l => l.Name).ToListAsync(ct);
         var now = DateTime.UtcNow;
-        foreach (var name in Defaults.Where(n => !existing.Contains(n)))
+        foreach (var name in Defaults)
         {
-            ctx.Lists.Add(new ListEntity
-            {
-                Id = Guid.NewGuid().ToString(),
-                Name = name,
-                CreatedAt = now,
-            });
+            var id = Guid.NewGuid().ToString();
+            // Atomic conditional insert: the SELECT ... WHERE NOT EXISTS is a single
+            // SQLite statement and cannot race — only one writer holds the lock.
+            await ctx.Database.ExecuteSqlAsync(
+                $"""
+                INSERT INTO lists (id, name, created_at, default_commit_type, sort_order)
+                SELECT {id}, {name}, {now}, 'chore', 0
+                WHERE NOT EXISTS (SELECT 1 FROM lists WHERE name = {name})
+                """, ct);
         }
-        await ctx.SaveChangesAsync(ct);
     }
 }