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.

shopdb/plugins/alembic_template.py

@@ -85,21 +85,24 @@ def _get_plugin_metadata(plugin_name: str) -> MetaData:
 
 
 def create_plugin_tables(plugin_name: str):
-    """Emit CreateTable DDL for every table this plugin owns. Idempotent
-    via Alembic's batch_op.create_table behavior (raises if exists; the
-    baseline migration is meant to run against an empty schema).
+    """Create every table this plugin owns, sourced from the SQLAlchemy models
+    (not duplicated DDL). IDEMPOTENT: a table that already exists is skipped, so
+    this is safe on an existing database that has the table from the pre-cutover
+    core baseline as well as on a fresh install (ADR-014 Phase 2).
 
-    Called from each plugin's 0001_baseline.py upgrade() so the table
-    definitions stay sourced from the SQLAlchemy models rather than being
-    duplicated in handwritten Alembic ops.
+    Called from each plugin's 0001 baseline.py upgrade().
     """
     from alembic import op
+    from sqlalchemy import inspect
     from sqlalchemy.schema import CreateTable
 
     md = _get_plugin_metadata(plugin_name)
     bind = op.get_bind()
+    existing = set(inspect(bind).get_table_names())
     # Sort by FK dependency so parent tables are created first.
     for table in md.sorted_tables:
+        if table.name in existing:
+            continue
         op.execute(str(CreateTable(table).compile(dialect=bind.dialect)))