Write down the tool that was built, not the one that was tabled

Secure Send shipped on 21 August and nothing outside the code caught up. The
roadmap still listed it under Deferred as "tabled", and gave as the reason that
the tools plugin is browser-only by design with get_blueprint and get_models
both empty - which stopped being true in the same day's commits that gave it a
blueprint, a table, a migration chain, a permission and a setting. A reader
following the roadmap would have concluded the feature does not exist.

The generated API docs were fine: api-inventory.json and openapi.json both
carry /api/tools/secrets, so the contract gates did their job. What was missing
was every word written for a person, and that is the half where the security
properties live.

docs/SECURE-SEND.md is the new page. The parts worth having in prose rather
than inferring from the code: that a lost secret is lost to everybody including
an administrator, which is the same property as "a compromised database yields
nothing" seen from the other side; that the 4096-character limit is the entire
server-side enforcement, because a server that cannot read what it holds cannot
have an opinion about content, so raising it turns a password tool into an
unaudited file transfer; that redeeming is a POST because preview prefetch in
chat and mail clients would otherwise spend a one-view secret before the
recipient ever sees it; and that somebody should send a link to themselves
through their own mail or chat client and confirm the key survives after the #
before a real credential goes through it, because a product that rewrites links
can strip the fragment and nothing on this side can help.

securesend_allow_guests was in the database, on a settings page, and in no
document. It is now in CONFIG.md with what it actually does - an unauthenticated
write endpoint, rate limited per address, failing closed - rather than only its
one-line description.

Also corrected along the way: site_base_url was documented as scheme + host
while the code deliberately tolerates a subpath in it, and the bundled plugin
table in PLUGINS.md had no row for tools at all.

Still missing after this: PLUGINS.md has no row for backups either.

docs/PLUGINS.md

@@ -20,6 +20,7 @@ These plugins are in `plugins/` in this repo. Enable per site with `flask plugin
 | `knowledgebase` | Knowledge Base articles linking to external resources | Lightweight article store. |
 | `printedparts` | 3D-printed parts inventory | Kiosk checkout / check-in. Ships `default_enabled: false`. |
 | `slides` | Slides for the lobby display and shop-floor screensaver | Upload / reorder / delete per surface. Management gated on the `slides.manage` permission, grantable to non-admin curators; display routes are public. |
+| `tools` | Technician utilities: barcode and QR label generation, unit conversions, Secure Send | Mostly browser-side by design, so an air-gapped bench gets them with no network. Secure Send is the exception and is why this plugin has a blueprint, a table and a migration chain: see [SECURE-SEND.md](SECURE-SEND.md). |
 | `warranty` | Asset warranty tracking | Manual entry now, Dell / Lenovo / HP provider lookups later. Derived coverage status with report buckets. |
 
 ## Plugin permissions