advisory-scan: separate benign source absence from real failures; fix pgautoupgrade URLs

Two refinements found by running the scan across all 14 recipes of the 2026-08-07 run:

1. A repo with no advisory feed returns HTTP 404 on /security-advisories (e.g. the
   pgautoupgrade sidecar image). That is a BENIGN ABSENCE, not a failed check.
   Likewise registry entries that are TEMPLATE urls for humans
   (…/changelog/v<VERSION>/, …/<vX.Y.Z>/…) are documentation, not fetchable.
   Counting either as a failure pushed most recipes to '?', which would make the
   unknown-vs-clean distinction meaningless again — the exact signal the ? exists to
   preserve. Both are now recorded in sources_benign; only genuine errors (rate
   limit, network, 5xx, wrong URL) land in sources_failed.

2. upstream/*.md pointed at github.com/pgautoupgrade/pgautoupgrade, which 404s —
   the repo is pgautoupgrade/docker-pgautoupgrade. Corrected in n8n, lasuite-docs,
   lasuite-drive, lasuite-meet. A 404ing registry URL means we were not scanning a
   source we believed we were.

Effect on the 2026-08-07 data: recipes with genuine failed sources 5 -> 3 (the
remainder are really unreachable vendor pages). CVE counts unchanged where they
were already sound: discourse 130, gitea 2, plausible 1.

cc-ci-plan/upstream/n8n.md

@@ -3,7 +3,7 @@
 | service | image | source repo | releases / changelog |
 |---------|-------|-------------|----------------------|
 | app     | n8nio/n8n | https://github.com/n8n-io/n8n | https://github.com/n8n-io/n8n/releases |
-| db      | pgautoupgrade/pgautoupgrade | https://github.com/pgautoupgrade/pgautoupgrade | https://hub.docker.com/r/pgautoupgrade/pgautoupgrade/tags |
+| db      | pgautoupgrade/pgautoupgrade | https://github.com/pgautoupgrade/docker-pgautoupgrade | https://hub.docker.com/r/pgautoupgrade/pgautoupgrade/tags |
 
 ## Standing notes
 - pgautoupgrade uses a non-standard tag scheme (e.g. `17-alpine`, `18-alpine`) mapping to the TARGET