ADR-014: schema-lean per-site (investigation + idempotent create_plugin_tables)

Cross-plugin FK blocker ADR-013 cited is already resolved: the FKs into
machines were held only by dead legacy tables (machinerelationships,
printerdata, installedapps, communications.machineid) that existing
migrations 7a01/7c01 already drop. No live plugin table hard-FKs another
plugin. Schema-lean is unblocked.

Enabling change: create_plugin_tables now skips already-existing tables
(idempotent) so a plugin anchor can create its tables on a fresh lean
install and no-op on a database that has them from the pre-cutover
baseline. The load-bearing baseline lift is staged as ADR-014 Phase 2.

tools/export-github.sh

@@ -44,6 +44,8 @@ rsync -a --delete \
   --exclude 'venv' \
   --exclude 'node_modules' \
   --exclude 'frontend/dist*' \
+  --exclude 'frontend/src/.plugins-staged' \
+  --exclude 'frontend/src/router/routes.gen.js' \
   --exclude 'instance' \
   --exclude '.env' \
   --exclude '__pycache__' \