Let Guides reach a site, and reach core the way every plugin must

Two faults the guides plugin shipped with, both caught by gates that were
already there.

deploy/site-profile-universal.json never listed `guides`. That file is the
profile the released Windows installer is built from, so the wizard would not
have offered Guides at all - and worse, `flask plugin prune-schema` drops the
tables of any plugin absent from the profile at provisioning (ADR-014). A site
installing the next release would have had its guides table dropped.

plugins/guides/models/guide.py imported shopdb.core.models.base directly.
Plugins reach core only through shopdb.api or shopdb.plugins.base (ADR-001),
and shopdb.api already re-exports BaseModel, so this is the same symbol by the
supported path.

The suite was red on main because of these two; it passes again.

deploy/site-profile-universal.json

@@ -5,6 +5,7 @@
     "computers",
     "employees",
     "geenforce",
+    "guides",
     "knowledgebase",
     "machines",
     "measuringtools",

plugins/guides/models/guide.py

@@ -33,8 +33,7 @@ without an author thinking about it.
 import json
 from datetime import datetime
 
-from shopdb.api import db
-from shopdb.core.models.base import BaseModel
+from shopdb.api import db, BaseModel
 
 
 def _utcnow():