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/classify/classify_test.go

@@ -61,6 +61,7 @@ func TestClassifyMatches(t *testing.T) {
 		{"x.s3-accesspoint-fips.us-east-1.amazonaws.com", want{model.ProviderAWS, model.ServiceS3, "x", "us-east-1", true, 0}},
 		{"x.mrap.accesspoint.s3-global.amazonaws.com", want{model.ProviderAWS, model.ServiceS3, "x", "", true, 0}},
 		{"bucket.s3express-zone1.eu-west-1.amazonaws.com", want{model.ProviderAWS, model.ServiceS3, "bucket", "eu-west-1", true, 0}},
+		{"bucket-01.s3-eu-west-1.amazonaws.com", want{model.ProviderAWS, model.ServiceS3, "bucket-01", "eu-west-1", true, 0}},
 		// AWS databases
 		{"mydb.cluster-abc.us-east-1.rds.amazonaws.com", want{model.ProviderAWS, model.ServiceRDS, "mydb", "us-east-1", true, 0}},
 		{"mydb.cluster-ro-abc.us-east-1.rds.amazonaws.com", want{model.ProviderAWS, model.ServiceRDS, "mydb", "us-east-1", true, 0}},