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.

migrations/0001_schema.sql

@@ -19,6 +19,11 @@ CREATE TABLE IF NOT EXISTS cloud_assets (
 
 CREATE INDEX IF NOT EXISTS cloud_assets_last_seen_idx ON cloud_assets (last_seen);
 
+-- Reachability probe results (added after the initial schema; ALTER keeps this
+-- idempotent for existing databases).
+ALTER TABLE cloud_assets ADD COLUMN IF NOT EXISTS exposure_detail TEXT        NOT NULL DEFAULT '';
+ALTER TABLE cloud_assets ADD COLUMN IF NOT EXISTS probed_at       TIMESTAMPTZ;
+
 CREATE TABLE IF NOT EXISTS observations (
     id              BIGSERIAL PRIMARY KEY,
     asset_id        BIGINT      NOT NULL REFERENCES cloud_assets (id) ON DELETE CASCADE,