add read-only web UI and per-vendor readability prober

Web UI (internal/webui):
- dependency-free embedded HTML/JS served on the loopback listener
- /api/assets (provider/service/exposure/q filters) and /api/stats
- optional HTTP basic auth via UI_USER/UI_PASS

Prober (internal/probe):
- vendor-specific, unauthenticated, non-destructive checks:
  S3/R2/Spaces ListObjectsV2, GCS JSON object list, Azure ?comp=list,
  Firebase /.json?shallow=true, Supabase REST, and bare TCP connect for
  database endpoints (never reported readable, no auth attempted)
- exposure_detail records the factual reason (status code, region, tcp port)
- bounded random-sample queue instead of FIFO: one high-volume hostname
  template (mrap) no longer starves every other asset type
- schema: cloud_assets.exposure_detail + probed_at (idempotent ALTER)

Live: stable ~450MiB under 1GiB, no restarts, UI and probe states
(private/reachable/auth_required) flowing, 0 db errors.

ARCHITECTURE.md

@@ -46,7 +46,7 @@ Certificate Transparency Logs
 | Direct queue | bound, non-blocking send | unbounded growth |
 | DNS enrichment | DNS via worker pool, TTL cache, CNAME follow | unbounded goroutines, persistence |
 | Dedup | memory-bounded TTL caches | unbounded maps |
-| Probe (optional) | DNS/TCP/TLS/HTTP HEAD/GET `/` | auth, downloads, mutation, DB or shell access |
+| Probe (optional) | one unauthenticated vendor read endpoint (S3/GCS/Azure/Firebase list) or a bare TCP connect for databases | credentials, object downloads, mutation, DB queries, auth attempts |
 | DB writer | batched `pgx.Batch` writes | one synchronous row per event |
 | Checkpoints | JSON CT position in `ct_checkpoints`/`ct_sths` | certificate/issuer contents in DB |