Add vulnerability scanning on top of the SBOM

The SBOM was generated and installed onto every server, and nothing read it.
"Are we exposed to this advisory" was still a person going and looking, which
meant it happened when somebody remembered to.

scripts/scan_sbom.py queries OSV.dev by purl - no account, no API key, no agent,
no vulnerability database to maintain on a box that cannot reach the internet.
The SBOM is already the input OSV wants, so this is a lookup against what
shipped rather than a second scan of the tree that could disagree with it.

Three decisions worth recording:

Shipped and build-only findings are both reported, but only shipped ones can
fail a build. Two thirds of the components here are build tooling the SBOM
already marks scope 'excluded', and the highest-severity match in the whole tree
today is a CRITICAL against the test runner. A gate that fails a release over
that trains everybody to bypass it, and then it catches nothing.

Severity is taken from the advisory, never derived. Where there is only a CVSS
vector the report says UNKNOWN, which is shown and never gates. A guessed score
is worse than an honest gap because somebody would act on it.

The allowlist is a record rather than a mute button: each entry carries the
reason, the accepter, the date, and what would invalidate the reasoning. The
first entry is a worked example - a PyJWT algorithm-confusion advisory that
needs both mixed algorithm families and a raw JSON JWK as the key, and this
application configures neither.

The report ends with a per-component upgrade rollup, since that is the part
somebody acts on. A target on a different major version is labelled rather than
listed beside patch bumps as though the two cost the same.

Current state: 8 shipped components have fixes available, none of them
exploitable through a path this application opens today.

CLAUDE.md

@@ -52,6 +52,13 @@ venv/bin/python scripts/gen_project_map.py --check
 A new plugin table means updating `PLUGIN_TABLE_OWNERS` and
 `EXPECTED_HEAD_REVISION` in `tests/test_plugin_migrations.py`.
 
+Dependencies have their own check. It needs the network, so it is not in the
+suite - run it before a release, and see `docs/DEPENDENCY-SECURITY.md`:
+
+```bash
+venv/bin/python scripts/scan_sbom.py --gate    # exit 1 on a SHIPPED finding
+```
+
 ## Database
 
 - **Active:** `shopdb_flask` (MySQL, asset-based schema)