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/pipeline/pipeline_test.go

@@ -149,7 +149,7 @@ func TestCNAMEMatch(t *testing.T) {
 	col := &collector{}
 	r := &fakeResolver{
 		cnames: map[string]string{"files.example.com": "files.s3.amazonaws.com"},
-		ips:    map[string][]net.IP{"files.example.com": {net.ParseIP("1.2.3.4")}},
+		ips:    map[string][]net.IP{"files.s3.amazonaws.com": {net.ParseIP("1.2.3.4")}},
 	}
 	m := metrics.New()
 	p := New(Config{Enricher: newEnricher(r), Write: col.write, Metrics: m, BatchSize: 10})
@@ -180,7 +180,7 @@ func TestCNAMEMatchIPv6(t *testing.T) {
 	col := &collector{}
 	r := &fakeResolver{
 		cnames: map[string]string{"db.example.com": "mydb.cluster-abc.us-east-1.rds.amazonaws.com"},
-		ips:    map[string][]net.IP{"db.example.com": {net.ParseIP("2606:4700::1111")}},
+		ips:    map[string][]net.IP{"mydb.cluster-abc.us-east-1.rds.amazonaws.com": {net.ParseIP("2606:4700::1111")}},
 	}
 	p := New(Config{Enricher: newEnricher(r), Write: col.write, BatchSize: 10})
 	ctx, cancel := context.WithCancel(context.Background())