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/Repositories/AppSettingsRepository.cs

@@ -18,8 +18,18 @@ public sealed class AppSettingsRepository
 
         row = new AppSettingsEntity { Id = AppSettingsEntity.SingletonId };
         _context.AppSettings.Add(row);
-        await _context.SaveChangesAsync(ct);
-        _context.Entry(row).State = EntityState.Detached;
+        try
+        {
+            await _context.SaveChangesAsync(ct);
+            _context.Entry(row).State = EntityState.Detached;
+        }
+        catch (DbUpdateException)
+        {
+            // Concurrent process already inserted the singleton — discard our attempt and re-read.
+            _context.Entry(row).State = EntityState.Detached;
+            row = await _context.AppSettings.AsNoTracking()
+                .FirstAsync(s => s.Id == AppSettingsEntity.SingletonId, ct);
+        }
         return row;
     }