✨ feat(user): magic-link expired-token cleanup loop (ADR-0028 Phase A consequence)

Periodically delete magic-link tokens past their expires_at — the rows
accumulate without cleanup since the consume endpoint only marks them
consumed but never removes them, and DeleteExpiredMagicLinkTokens (added
in Phase A.3, PR #61) was never wired up.

Mirrors the JWT secret cleanup pattern (ADR-0021).

- pkg/user/magic_link_cleanup.go: MagicLinkCleanupRunner with
  StartCleanupLoop(ctx, interval) and an inner runOnce(ctx) for testability
- pkg/user/magic_link_cleanup_test.go: unit tests cover happy path,
  error propagation, ctx-cancel termination, zero-interval no-op
- pkg/server/server.go: start the loop right after the JWT cleanup loop
- pkg/config/config.go: auth.magic_link.cleanup_interval (default 1h),
  env DLC_AUTH_MAGIC_LINK_CLEANUP_INTERVAL, getter GetMagicLinkCleanupInterval

Mostly authored by Mistral Vibe in batch1-task-b worktree (parallel with
batch1-task-a OIDC config). Mistral hit --max-price 1.50 right before the
final commit step, Claude finished the ship via trainer takeover (Q-045
pattern, 2nd recurrence in two batches — to be addressed in Phase 1bis).

pkg/server/server.go

@@ -764,6 +764,12 @@ func (s *Server) Run() error {
 		s.userService.StartJWTSecretCleanupLoop(rootCtx, s.config.GetJWTSecretCleanupInterval())
 	}
 
+	// Start the magic-link expired-token cleanup loop (ADR-0028 Phase A consequence).
+	if mlRepo, ok := s.userRepo.(user.MagicLinkRepository); ok {
+		runner := user.NewMagicLinkCleanupRunner(mlRepo)
+		runner.StartCleanupLoop(rootCtx, s.config.GetMagicLinkCleanupInterval())
+	}
+
 	// Wire the sampler hot-reload callback (ADR-0023 Phase 3, sub-phase 3.3).
 	// telemetrySetup is non-nil only when telemetry was successfully initialized
 	// at startup — hot-reloading telemetry-on is out of scope (see ADR-0023).