Phase 7B: per-plugin Alembic chains for bundled plugins

Each of the six bundled plugins (computers, equipment, network,
notifications, printers, usb) now has its own Alembic chain with a
baseline migration. Sister sites adopting one of these plugins can
manage its schema via `flask plugin migrate <name>` instead of relying
on db.create_all to bootstrap everything.

Existing single-site deploys that bootstrap via db.create_all continue
to work unchanged. The chains coexist; the bootstrap path stays the
operator's choice.

Framework
- shopdb/plugins/alembic_template.py: shared env.py logic + helpers.
  PLUGIN_TABLE_OWNERS pins which tables belong to which plugin (explicit
  registry, not import-side-effect). _get_plugin_metadata filters
  db.metadata to only the named plugin's tables. create_plugin_tables /
  drop_plugin_tables emit DDL via SQLAlchemy CreateTable so the table
  definitions stay sourced from the models, not duplicated.
- shopdb/plugins/__init__.py: PluginManager.upgrade_all_plugins() runs
  pending migrations across every discovered plugin and returns a status
  dict. Idempotent (Alembic skips applied revisions).

CLI
- `flask plugin upgrade-all` runs pending migrations for every plugin.
  Used on a fresh deploy after the core schema is in place.

Per-plugin scaffolding
- plugins/{computers,equipment,network,notifications,printers,usb}/
  migrations/{alembic.ini, env.py, script.py.mako, versions/0001_baseline.py}
- Each env.py is a 5-line shim that sets PLUGIN_NAME and delegates to
  the shared template. Each 0001_baseline calls create_plugin_tables(name)
  / drop_plugin_tables(name); no duplication of column definitions.

Tests
- tests/test_plugin_migrations.py (18 cases): every bundled plugin has
  an entry in PLUGIN_TABLE_OWNERS, has the on-disk Alembic scaffolding,
  and the filtered MetaData contains every owned table (catches drift
  between the template's table list and what the models declare).
- 129 tests pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/test_plugin_migrations.py

@@ -0,0 +1,51 @@
+"""Per-plugin Alembic chain wiring tests.
+
+Pins the bundled-plugin migration setup so a future refactor that breaks
+the scaffolding fails fast with a clear test rather than a confusing
+runtime error on a fresh deploy.
+"""
+from pathlib import Path
+
+import pytest
+
+from shopdb.plugins.alembic_template import (
+    PLUGIN_TABLE_OWNERS,
+    _get_plugin_metadata,
+)
+
+
+BUNDLED_PLUGINS = ('computers', 'equipment', 'network', 'notifications', 'printers', 'usb')
+
+
+@pytest.mark.parametrize('plugin', BUNDLED_PLUGINS)
+def test_bundled_plugin_has_table_owner_entry(plugin):
+    """Every bundled plugin appears in PLUGIN_TABLE_OWNERS with at least
+    one table; otherwise its baseline migration would be a no-op."""
+    assert plugin in PLUGIN_TABLE_OWNERS
+    assert len(PLUGIN_TABLE_OWNERS[plugin]) > 0
+
+
+@pytest.mark.parametrize('plugin', BUNDLED_PLUGINS)
+def test_bundled_plugin_has_migrations_dir(plugin):
+    """Each bundled plugin has the on-disk Alembic scaffolding."""
+    root = Path(__file__).resolve().parent.parent / 'plugins' / plugin / 'migrations'
+    assert (root / 'env.py').is_file(), f"{plugin}/migrations/env.py missing"
+    assert (root / 'alembic.ini').is_file(), f"{plugin}/migrations/alembic.ini missing"
+    assert (root / 'script.py.mako').is_file(), f"{plugin}/migrations/script.py.mako missing"
+    versions = root / 'versions'
+    assert versions.is_dir(), f"{plugin}/migrations/versions missing"
+    baseline = versions / '0001_baseline.py'
+    assert baseline.is_file(), f"{plugin}/migrations/versions/0001_baseline.py missing"
+
+
+@pytest.mark.parametrize('plugin', BUNDLED_PLUGINS)
+def test_plugin_metadata_has_all_owned_tables(plugin, app):
+    """The MetaData filtered to a plugin's owned tables actually contains
+    every table named in PLUGIN_TABLE_OWNERS. Catches drift between the
+    template's table list and what the models declare."""
+    with app.app_context():
+        md = _get_plugin_metadata(plugin)
+        owned = set(PLUGIN_TABLE_OWNERS[plugin])
+        present = set(md.tables.keys())
+        missing = owned - present
+        assert not missing, f"Plugin {plugin}: tables in PLUGIN_TABLE_OWNERS not in metadata: {missing}"