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/ClaudeDoDbContextModelSnapshot.cs

@@ -211,6 +211,10 @@ namespace ClaudeDo.Data.Migrations
 
                     b.HasKey("Id");
 
+                    b.HasIndex("Name")
+                        .IsUnique()
+                        .HasDatabaseName("idx_lists_name");
+
                     b.HasIndex("SortOrder")
                         .HasDatabaseName("idx_lists_sort");