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/PasskeyController.cs

@@ -20,6 +20,7 @@ namespace PowderCoating.Web.Controllers;
 /// matches automatically on localhost, dev, staging, and production without any
 /// environment-specific configuration.
 /// </summary>
+// Intentional exception: WebAuthn/FIDO2 identity infrastructure. UserPasskeys is an ASP.NET Identity concern not exposed through IUnitOfWork; the anonymous login path has no tenant context; FIDO2 async callbacks capture _db by closure. See docs/DATA_ACCESS_ARCHITECTURE.md — Permanent Exceptions.
 [Route("[controller]/[action]")]
 public class PasskeyController : Controller
 {