Phases 3 & 4: Complete data access architecture migration

Phase 3 — eliminated ApplicationDbContext from all non-exempt controllers,
routing all data access through IUnitOfWork. Added IPlainRepository<T> for
the four platform entities (Announcement, BannedIp, DashboardTip, ReleaseNote)
that intentionally don't extend BaseEntity and therefore can't use the
constrained IRepository<T>. Added permanent-exception comments to the 18
controllers that legitimately retain direct DbContext access (Identity infra,
cross-tenant platform ops, bulk streaming exports).

Phase 4 — added EnforceDataAccessArchitecture() to Program.cs, a startup
gate that reflects over every Controller subclass and throws at boot if any
non-exempt controller injects ApplicationDbContext. The app cannot start with
a violation.

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

src/PowderCoating.Web/Controllers/AuditLogController.cs

@@ -14,6 +14,7 @@ namespace PowderCoating.Web.Controllers;
 /// the application — they are append-only by design to maintain an unambiguous
 /// trail of changes across all tenants.
 /// </summary>
+// Intentional exception: platform audit log with a long PK; append-only infrastructure table outside the business entity graph; same reasoning as SystemLogsController. See docs/DATA_ACCESS_ARCHITECTURE.md — Permanent Exceptions.
 [Authorize(Policy = AppConstants.Policies.SuperAdminOnly)]
 public class AuditLogController : Controller
 {