provider capability model, on-demand scan/backfill, more providers

Readability capabilities (exists/object_readable/listable) are now tracked
separately instead of a single exposure flag, matching how providers differ:
- Azure 'Blob' mode = object readable, not listable; 'Container' = listable
- Cloudflare R2 public URLs are never anonymously listable (root 500); R2 is
  object-read only and is no longer probed for listing
- S3/GCS: 200 list = listable; 403 + readable index.html/robots.txt =
  object readable only
New columns: cloud_assets.listable, cloud_assets.object_readable.

Azure: account-level container listing is never anonymous; discover containers
by probing a bounded common-name set (or the container from a URL path), then
list blobs.

Classifier: added the legacy AWS '.s3-<region>.amazonaws.com' bucket form.

On-demand scan: POST /api/scan {target} classifies a host or URL, follows CNAME
for custom domains, probes, and indexes readable resources; Azure container
comes from the URL path. UI has a Scan box.

Backfill: POST /api/reprobe re-probes already-discovered assets with the current
checks and indexes any readable ones, recovering findings from earlier probes.

DNS: CNAME-only enrichment (A/AAAA resolved only for matches), 64 workers,
larger queue.

Verified live against real public resources:
  veteran1.nyc3.digitaloceanspaces.com            listable, entries
  bucket-01.s3-eu-west-1.amazonaws.com            listable, entries
  tartugeo-podcast.storage.googleapis.com         listable, entries
  eroticfeeldev.blob.core.windows.net/images      listable, entries

internal/store/store_test.go

@@ -60,7 +60,7 @@ func TestIntegration(t *testing.T) {
 	if err := s.WriteFindings(ctx, []pipeline.Finding{f}); err != nil {
 		t.Fatal(err)
 	}
-	if err := s.UpdateExposure(ctx, model.ProviderAWS.String(), model.ServiceS3.String(), f.Hostname, model.ExposureReachable, "test"); err != nil {
+	if err := s.UpdateExposure(ctx, model.ProviderAWS.String(), model.ServiceS3.String(), f.Hostname, model.ExposurePublicRead, "test", true, true); err != nil {
 		t.Fatal(err)
 	}
 	if _, err := s.ListAssets(ctx, AssetFilter{Provider: "aws", Limit: 10}); err != nil {